Heterojunction Bipolar Transistor

Abstract

This heterojunction bipolar transistor is provided with: a substrate (101) formed from InP; a subcollector layer (102) formed on the substrate (101); a collector layer (103); a base layer (104); an emitter layer (105); and an emitter cap layer (106). The base layer (104) is formed from a first base layer (104a) located on the collector layer (103) side and a second base layer (104b) located on the emitter layer (105) side. The Sb molar composition ratio of the first base layer (104a) is constant in the thickness direction. In the second base layer (104b), the Sb molar composition ratio is higher at a location closer to the emitter layer (105).

Description

[Technical field]

[0001] The present invention relates to a heterojunction bipolar transistor constructed from III-V compound semiconductors. [Background technology]

[0002] In recent years, with the advancement of IoT (Internet of Things) and cloud computing, the demand for faster and larger capacity communication has been increasing rapidly. In order to meet this demand in communication, the use of frequency bands around 1 THz, called millimeter waves and terahertz bands, has been considered, and some of them have already been put to practical use. In order to use frequency bands around 1 THz, electronic circuits that operate at high speeds are required. Hetero-junction bipolar transistors on InP substrates have excellent high-frequency characteristics and have been actively researched as electronic devices to be used in electronic circuits that operate at high speeds (see, for example, Non-Patent Document 1).

[0003] The layer structure of heterojunction bipolar transistors can be broadly classified into single heterojunction bipolar transistors (SHBTs), in which the same semiconductor material is used for the base layer and collector layer and a semiconductor material with a large band gap is used for the emitter layer, and double heterojunction bipolar transistors (DHBTs), in which semiconductor materials with a larger band gap than the base layer are used for not only the emitter layer but also the collector layer.

[0004] Single heterojunction bipolar transistors have the advantage that the layer structure is relatively easy to fabricate and that there is no potential barrier between the base layer and the emitter layer that would prevent electron movement, but they have the problem of low collector breakdown voltage due to the small band gap of the material used in the collector layer. On the other hand, double heterojunction bipolar transistors can use materials with a large band gap for the collector layer, so the problem of low collector breakdown voltage mentioned above can be avoided, but another problem exists. This problem will be explained with reference to Figures 15, 16, and 17. As examples of double heterojunction bipolar transistors fabricated on an InP substrate, we will explain the cases where InGaAs and GaAsSb are used for the base layer.

[0005] FIG. 15 is a schematic diagram showing the band arrangement in a thermal equilibrium state (at zero bias) when the base layer 304 is made of InGaAs doped with a high concentration of p-type impurities, and the collector layer 303 and the emitter layer 305 are made of InP doped with a low concentration of n-type impurities.

[0006] FIG. 16 is a schematic diagram showing the band arrangement in a thermal equilibrium state (at zero bias) when the base layer 304 is made of GaAsSb doped with a high concentration of p-type impurities, and the collector layer 303 and the emitter layer 305 are made of InP doped with a low concentration of n-type impurities.

[0007] First, the problem when the base layer 304 is made of InGaAs will be described. A double heterostructure in which the base layer 304 made of InGaAs is sandwiched between the collector layer 303 and the emitter layer 305 made of InP has a type I band arrangement as shown in Fig. 15, and the energy position of the bottom of the conduction band is higher in the collector layer 303 than in the base layer 304. In this case, a potential barrier exists when electrons move from the base layer 304 to the collector layer 303. This potential barrier is called a band discontinuity in the conduction band.

[0008] Generally, even if there is a small potential barrier in the conduction band at room temperature, it does not become a major obstacle to the electron movement. Specifically, if the band discontinuity of the conduction band is about 0.1 eV, which is about four times the product of the Boltzmann constant and the temperature (kBT=26 meV at 300 K), the electron movement is not easily affected by the band discontinuity.

[0009] However, when the base layer 304 is made of InGaAs (band gap = 0.75 eV) that is lattice-matched to InP, the band discontinuity in the conduction band is 0.2 eV or more. This causes so-called current blocking, which suppresses the movement of electrons from the base layer 304 to the collector layer 303. For this reason, when the base layer 304 is made of InGaAs, the stacked structure of the collector layer 303, the base layer 304, and the emitter layer 305 is rarely used as a double heterojunction bipolar transistor.

[0010] The potential barrier when electrons move from the base layer to the collector layer can be reduced by making the collector layer from a semiconductor material with a smaller band gap than InP. However, in this case, although it is an improvement over a single heterojunction bipolar transistor, the collector layer still has the problem of being unable to withstand the applied voltage.

[0011] These problems that arise when InGaAs is used for the base layer can be solved by using GaAsSb for the base layer, as will be explained next.

[0012] The band arrangement shown in FIG. 16 is a double heterostructure in which a base layer 304 made of GaAsSb is sandwiched between a collector layer 303 and an emitter layer 305 made of InP, and this layer configuration has a type II band arrangement. In this case, the band discontinuity of the conduction band at the interface between the emitter layer 305 and the base layer 304 becomes a potential barrier when electrons move. The band discontinuity between the emitter layer 305 and the base layer 304 made of GaAsSb is smaller than the band discontinuity between the base layer made of InGaAs and the emitter layer made of InP, and the potential barrier against electron movement is smaller. In addition, the potential barrier between the emitter layer 305 and the base layer 304 made of GaAsSb can be reduced by replacing the emitter layer 305 with a material having a large band gap, such as InGaP, InAlP, or InAlAs, from InP.

[0013] That is, in the layer structure having the band arrangement of Fig. 16, even if the emitter layer 305 is replaced with a material having a larger band gap than InP, the withstand voltage against the application of voltage does not decrease. Therefore, by replacing the base layer of the double heterojunction bipolar transistor from InGaAs to GaAsSb, the problem of the potential barrier that prevents the movement of electrons can be solved and the withstand voltage against the application of voltage can be secured.

[0014] From the above, it is expected that double heterojunction bipolar transistors with GaAsSb as the base layer will have improved device characteristics, specifically, improved current gain cutoff frequency, compared to those using InGaAs. However, the current gain cutoff frequency of devices actually fabricated is not significantly different from those using InGaAs as the base layer, which means that the advantages of the band arrangement are not being utilized. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] JP 2003-086602 A [Patent Document 2] JP 2011-009330 A [Non-patent literature]

[0016] [Non-Patent Document 1] CR Bolognesi et al., "InP / GaAsSb DHBTs for THz Applications and Improved Extraction of their Cutoff Frequencies", IEEE International Electron Devices Meeting, 723-726, 2016. [Non-Patent Document 2] CR Bolognesi et al., "InP / GaAsSb / InP Double HBTs: A New Alternative for InP-Based DHBTs", IEEE Transactions on Electron Devices, vol. 48, no. 11, pp. 2631-2639, 2001. [Non-Patent Document 3] M. Peter et al., "Band gaps and band offsets in strained GaAs1-ySby on InP grown by metalorganic chemical vapor deposition", Applied Physics Letters, vol. 74, no. 3, pp. 410-412, 1999. [Non-Patent Document 4] JYT Huang et al., "Characteristics of strained GaAs1-y Sby (0.16≦y≦0.69) quantum wells on InP substrates", JOURNAL OF PHYSICS D: APPLIED PHYSICS, vol. 40, pp. 7656-7661, 2007. Summary of the Invention [Problem to be solved by the invention]

[0017] The reasons why double heterojunction bipolar transistors with GaAsSb as the base layer have not achieved the expected device characteristics are thought to be due to material problems with GaAsSb and the effect of the potential barrier between the base and collector layers on electron movement. The material problems and the effect of the potential barrier are explained below.

[0018] First, the material problem of GaAsSb will be described. It is known that a p-type GaAsSb layer has a lower hole mobility than a p-type InGaAs layer even if the doping amount is the same as that of a p-type InGaAs layer (see, for example, Patent Document 1). For this reason, when GaAsSb is used for the base layer, in order to obtain the same base sheet resistance as when InGaAs is used, GaAsSb must be doped with p-type impurities at a higher concentration than InGaAs.

[0019] Specifically, when InGaAs is used for the base layer, 19 cm -3 Although high p-type impurity concentrations are often used in semiconductors with similar base sheet resistance, an even higher p-type impurity concentration is required for GaAsSb. However, doping GaAsSb with a high concentration of p-type impurities makes it difficult to ensure crystal growth and reliability.

[0020] Furthermore, GaAsSb has a mobility problem. In a heterojunction bipolar transistor, a factor related to the current gain cutoff frequency is the mobility of electrons in the base layer doped with a high concentration of p-type impurities. The smaller this mobility is, the longer it takes for electrons to pass through the base layer, and the lower the current gain cutoff frequency is. The mobility of electrons in a layer doped with p-type impurities cannot be measured directly from an experiment, but an approximate value can be obtained by analyzing device characteristics. Specifically, it has been reported that the electron mobility of C-doped p-type GaAsSb, which was obtained by analyzing device characteristics, is about 1 / 4 to 1 / 5 of the electron mobility of p-type InGaAs (see Non-Patent Document 2).

[0021] As described above, when GaAsSb is used for the base layer, it is necessary to dope it with a very high p-type impurity, and further, there are problems with the material of GaAsSb, such as its low mobility.

[0022] Next, the effect of the potential barrier between the base layer and the collector layer on electron movement will be described. The band arrangement of the double heterojunction bipolar transistor shown in FIG. 16 is in a thermal equilibrium state without applying a bias voltage, but a bias voltage is applied during operation. FIG. 17 is a schematic diagram showing the band arrangement when a bias voltage is applied to the layer structure of FIG. 16. Since the space charge in the collector layer 303 is not compensated during operation, if the band discontinuity between the base layer 304 and the collector layer 303 is large, band curvature occurs in the collector layer 303, and a potential notch structure is formed at the bottom of the conduction band (see, for example, Patent Document 2). When electrons accumulate due to this potential notch structure, the transit time of electrons in the collector layer increases, and as a result, the current gain cutoff frequency decreases.

[0023] To reduce the effect of this potential notch structure, it is necessary to change the Sb molar composition ratio of GaAsSb to reduce the band discontinuity between the base layer 304 and the collector layer 303. To achieve this, it is necessary to correctly understand the change in the band discontinuity in the conduction band between GaAsSb and InP due to the Sb molar composition ratio of GaAsSb.

[0024] However, even now, it is difficult to calculate the bottom energy of the conduction band when the Sb molar composition ratio of GaAsSb is changed (see, for example, Non-Patent Documents 3 and 4). Therefore, it is difficult to quantitatively determine the Sb molar composition ratio of GaAsSb that can reduce the band discontinuity in the conduction bands of GaAsSb and InP near room temperature, which is the operating temperature. For this reason, there is currently a problem that heterojunction bipolar transistors using GaAsSb as the base layer do not provide the expected device characteristics.

[0025] The present invention has been made to solve the above problems, and has as its object to provide a heterojunction bipolar transistor using GaAsSb as the base layer that can achieve the expected device characteristics. [Means for solving the problem]

[0026] The heterojunction bipolar transistor according to the present invention includes a substrate made of InP, a collector layer formed on the substrate and made of a III-V compound semiconductor, and a semiconductor layer formed on the collector layer, GaAsSb and an emitter layer formed on the base layer and made of a III-V compound semiconductor different from that of the base layer. The Sb molar composition ratio of the base layer decreases in the thickness direction from the emitter layer side to halfway through the base layer and is constant from halfway through the base layer to the collector layer. Effect of the Invention

[0027] As described above, according to the present invention, the Sb molar composition ratio of the base layer made of a III-V compound semiconductor containing Ga, As, and Sb decreases in the thickness direction from the emitter layer side to the middle of the base layer and is constant from the middle of the base layer to the collector layer. Therefore, a heterojunction bipolar transistor using GaAsSb for the base layer can obtain expected device characteristics. [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 is a cross-sectional view showing a configuration of a heterojunction bipolar transistor according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a band diagram showing a band arrangement in a thermal equilibrium state around base layer 104 in the layer structure of the heterojunction bipolar transistor according to the embodiment. [Diagram 3] FIG. 3 is a characteristic diagram showing the change in the miscibility gap of InGaAsSb depending on the growth temperature. [Figure 4]FIG. 4 is a band diagram showing the band arrangement of a heterostructure of GaAsSb and InP. [Diagram 5] FIG. 5 is a characteristic diagram showing the results of determining the interband transitions between types I and II at 300K. [Figure 6] FIG. 6 is a characteristic diagram showing the change in band discontinuity in the conduction band of a GaAsSb / InP heterostructure at 300K depending on the Sb molar composition ratio of GaAsSb. [Figure 7] FIG. 7 is a characteristic diagram showing the change in band discontinuity in the conduction band between an emitter layer made of In0.8Ga0.2P and a base layer made of GaAsSb depending on the Sb molar composition ratio of GaAsSb. [Figure 8] FIG. 8 is a characteristic diagram showing the band discontinuity of the conduction band between InGaP and GaAsSb when the Ga molar composition ratio of InGaP is changed to 0, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30. [Figure 9] FIG. 9 is a characteristic diagram showing the change in lattice strain depending on the Ga molar composition ratio of InGaP on InP. [Figure 10] FIG. 10 is a characteristic diagram showing the change in lattice strain depending on the Sb molar composition ratio of GaAsSb on InP. [Figure 11] FIG. 11 is a characteristic diagram showing the change in lattice strain in the layers when the Sb molar composition ratio in the GaAsSb base layer and the Ga molar composition ratio in the InGaP emitter layer are changed. [Figure 12] FIG. 12 is a characteristic diagram showing the measurement results of the X-ray diffraction pattern of a sample in which GaAsSb is grown on InP with a tensile strain of 1% (Sb molar composition ratio 0.36) and the thickness is changed. [Figure 13] FIG. 13 is a characteristic diagram showing the results of microscopic PL mapping measurements of GaAsSb samples grown to thicknesses of 35 nm and 46 nm with a tensile strain of 1%. [Figure 14] FIG. 14 is a band diagram showing the energy position of the bottom of the conduction band when a GaAsSb base layer having the structure described with reference to FIG. 11 is used. [Figure 15]FIG. 15 is a band diagram showing a band arrangement in a thermal equilibrium state when base layer 304 is made of InGaAs doped with a high concentration of p-type impurities, and collector layer 303 and emitter layer 305 are made of InP doped with a low concentration of n-type impurities. [Figure 16] FIG. 16 is a band diagram showing a band arrangement in a thermal equilibrium state when the base layer 304 is made of GaAsSb doped with a high concentration of p-type impurities, and the collector layer 303 and the emitter layer 305 are made of InP doped with a low concentration of n-type impurities. [Figure 17] FIG. 17 is a band diagram showing the band arrangement when a bias voltage is applied when the base layer 304 is made of GaAsSb doped with a high concentration of p-type impurities, and the collector layer 303 and the emitter layer 305 are made of InP doped with a low concentration of n-type impurities. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Hereinafter, a heterojunction bipolar transistor according to an embodiment of the present invention will be described with reference to FIG.

[0030] This heterojunction bipolar transistor includes a substrate 101 made of InP, and a subcollector layer 102, a collector layer 103, a base layer 104, an emitter layer 105, and an emitter cap layer 106 formed on the substrate 101. This heterojunction bipolar transistor is a so-called double heterojunction bipolar transistor.

[0031] The sub-collector layer 102 is formed on the substrate 101 and is made of a III-V compound semiconductor. The sub-collector layer 102 may have a laminated structure of, for example, an InP layer having a thickness of 200 nm and an InGaAs layer having a thickness of 100 nm. The InP layer has an n-type carrier concentration of 5×10 19 cm -3 The InGaAs layer has an n-type carrier concentration of 3×10 19 cm -3and the In molar composition ratio can be set to 0.53.

[0032] The collector layer 103 is formed on the sub-collector layer 102 and is made of a III-V compound semiconductor. The collector layer 103 is made of InP and has a thickness of, for example, 100 nm and an n-type carrier concentration of 3×10 16 cm -3 It can be said that:

[0033] The base layer 104 is formed on the collector layer 103 and is made of a III-V compound semiconductor containing Ga, As, and Sb. The base layer 104 may be made of GaAsSb. Here, the base layer 104 is made of a first base layer 104a on the collector layer 103 side and a second base layer 104b on the emitter layer 105 side. The first base layer 104a has a constant Sb molar composition ratio in the thickness direction. The second base layer 104b has an Sb molar composition ratio that increases in the thickness direction toward the emitter layer 105.

[0034] In this configuration, the Sb molar composition ratio of base layer 104 decreases in the thickness direction from the emitter layer 105 side to the middle of base layer 104, and is constant from the middle of base layer 104 to collector layer 103. For example, the Sb molar composition ratio of base layer 104 can be in the range of 0.49 to 0.53 in the vicinity of the interface with emitter layer 105 in the thickness direction, and in the range of 0.3 to 0.4 in the vicinity of the interface with collector layer 103. The thickness of base layer 104 can be 35 nm or less.

[0035] For example, the first base layer 104a has a thickness of 10 nm and a p-type carrier concentration of 6×10 19 cm -3 The second base layer 104b has a thickness of 20 nm and a p-type carrier concentration of 6×10 19 cm -3 The Sb molar composition ratio may be configured to continuously increase from 0.36 to 0.49 toward the emitter layer 105 side.

[0036] The emitter layer 105 is formed on the base layer 104 and is made of a III-V compound semiconductor different from the base layer 104. The emitter layer 105 may be made of an InGaP layer 105a made of InGaP in a part of its thickness direction, and an upper emitter layer 105b formed on the InGaP layer 105a. The Ga molar composition ratio of the InGaP layer 105a may be configured to be greater than 0 and less than or equal to 0.25, and to increase toward the base layer 104.

[0037] For example, the InGaP layer 105a has a thickness of 10 nm and an n-type carrier concentration of 3×10 17 cm -3 The Ga molar composition ratio may be continuously decreased from 0.20 to 0 toward the upper emitter layer 105b. The upper emitter layer 105b may be made of InP, have a thickness of 10 nm, and have an n-type carrier concentration of 3×10 17 cm -3 It can be said that:

[0038] The emitter cap layer 106 is formed on the emitter layer 105 and is made of a III-V compound semiconductor. The emitter cap layer 106 is made of, for example, InGaAs, has a thickness of 200 nm, and has an n-type carrier concentration of 3×10 19 cm -3 The In molar composition ratio can be set to 0.53.

[0039] Moreover, the collector layer 103 and the base layer 104 are formed in a predetermined mesa structure (collector mesa), and a collector electrode 111 is formed on the sub-collector layer 102 around the mesa structure. The collector electrode 111 is in ohmic contact with the sub-collector layer 102 and is electrically connected to the collector layer 103. Moreover, the emitter layer 105 and the emitter cap layer 106 are formed in a predetermined mesa structure (emitter mesa), and a base electrode 112 is formed on the base layer 104 (second base layer 104b) around the mesa structure. The base electrode 112 is in ohmic contact and electrically connected to the base layer 104 (second base layer 104b). Moreover, an emitter electrode 113 is formed on the emitter cap layer 106, and is in ohmic contact with the emitter cap layer 106.

[0040] For example, the above-mentioned layers are epitaxially grown in sequence on the substrate 101 by a well-known metalorganic vapor phase epitaxy method. Note that the method is not limited to metalorganic vapor phase epitaxy, and each layer can be epitaxially grown by a method such as molecular beam epitaxy, metalorganic molecular beam epitaxy, or gas source molecular beam epitaxy. Next, an emitter electrode material is deposited on the emitter cap layer 106 to form a metal film. Next, the metal film is patterned by a known lithography technique to form the emitter electrode 113.

[0041] Next, the emitter cap layer 106 and the emitter layer 105 are selectively etched by a known etching technique using the formed emitter electrode 113 as a mask to form an emitter mesa. The dimensions of the emitter mesa in a plan view can be 0.5 μm×2 μm. For example, first, the emitter layer 105 is etched close to the InGaP layer 105a. Next, the pattern thus formed is covered with a protective film made of an insulating material, and then completely etched down to the InGaP layer 105a to expose the second base layer 104b, thereby forming the emitter mesa.

[0042] After forming the emitter mesa as described above, while leaving the protective film, a base electrode material is deposited on top of it to form a metal film, and this metal film is patterned by a known lift-off method by removing the protective film to form the base electrode 112.

[0043] Next, using known lithography and etching techniques, the base layer 104 and the collector layer 103 are patterned to form a collector mesa, and a region in which the sub-collector layer 102 is exposed is formed on the side of the collector mesa. Next, a collector electrode 111 is formed in the region in which the sub-collector layer 102 is exposed. Finally, elements are isolated using known etching techniques, thereby completing the fabrication of a heterojunction bipolar transistor.

[0044] According to the above-described embodiment, the problem of electron migration from the base layer made of GaAsSb to the heterojunction collector layer is solved, and the expected device characteristics can be obtained in a heterojunction bipolar transistor using GaAsSb for the base layer.

[0045] 2 shows the band arrangement in a thermal equilibrium state around base layer 104 in the layer structure of the heterojunction bipolar transistor according to the embodiment described above. The energy difference at the bottom of the conduction band is about 60 meV between InGaP layer 105a and second base layer 104b, and about 40 meV between first base layer 104a and collector layer 103, which is smaller than the energy difference of 100 meV at the bottom of the conduction band at which a potential barrier or a potential notch structure in electron transport becomes a problem.

[0046] The current gain cutoff frequency of the heterojunction bipolar transistor according to the embodiment described above is 320 GHz when the collector-emitter bias voltage is 1.2 V. For comparison, the base layer is 30 nm thick and the p-type carrier concentration is 6×10 19 cm -3In a comparative heterojunction bipolar transistor made only of GaAsSb with the Sb molar composition ratio continuously increased from 0.36 to 0.49, the current gain cutoff frequency is 280 GHz.

[0047] The reason why the current gain cutoff frequency of the embodiment is higher than that of the comparative heterojunction bipolar transistor is believed to be due to the shortened transit time of electrons in the base layer. Thus, according to the embodiment, the current gain cutoff frequency of the heterojunction bipolar transistor can be increased.

[0048] As described above, according to the first embodiment, the Sb molar composition ratio of the base layer made of GaAsSb decreases in the thickness direction from the emitter layer side to the middle of the base layer and is constant from the middle of the base layer to the collector layer 103. Therefore, this type of heterojunction bipolar transistor can obtain expected device characteristics.

[0049] This invention aims to reduce the effect on electron mobility of the problem that occurs when GaAsSb is used in the base layer of a double heterojunction bipolar transistor, thereby drawing out the potential of the base layer made of GaAsSb and facilitating the improvement of device characteristics. In the following, we clarify the relationship between the band alignment in the conduction band of GaAsSb and InP at room temperature and the Sb molar composition ratio of GaAsSb, which has been difficult to quantitatively obtain until now.

[0050] As mentioned above, one of the reasons why it is difficult to improve the device characteristics of double heterojunction bipolar transistors with GaAsSb as the base layer is that the electron mobility of GaAsSb is small. The electron mobility of GaAsSb is closely related to the Sb molar composition ratio. This will be explained first.

[0051] GaAsSb lattice-matches to InP when the Sb molar composition ratio is approximately 0.49. For this reason, when GaAsSb is used in devices on an InP substrate, a value close to 0.49 is often used as the Sb molar composition ratio of GaAsSb. However, it is known that compositional separation is likely to occur in GaAsSb with an Sb molar composition ratio close to the condition for lattice matching with InP (see, for example, References 1 and 2). This compositional separation is influenced by the miscibility gap (see, for example, Reference 3).

[0052] Figure 3 shows how the miscibility gap of InGaAsSb changes with growth temperature. Figure 3 shows that when the InGaAsSb composition is inside the miscibility gap, compositional separation is likely to occur. The composition on the right axis of Figure 3 corresponds to GaAsSb. In InGaAsSb, the lattice constant changes depending on the molar composition ratio of Ga and Sb, and the lattice strain applied to the crystal changes. The diagonal lines in the figure show the contours of the lattice strain applied to InGaAsSb, from -1.5% (tensile strain) to +1.0% (compressive strain).

[0053] From Figure 3, it can be seen that the miscibility gap becomes smaller as the growth temperature is increased from 500°C to 550°C and 600°C. Therefore, the effect of compositional segregation can be reduced by increasing the growth temperature. However, the compositional region of GaAsSb close to the condition for lattice matching with InP (compositional region with lattice strain close to 0%) is located almost in the center of the miscibility gap even when the growth temperature is set to 600°C, so it is difficult to avoid the effect of compositional segregation. Due to the effect of compositional segregation, in GaAsSb close to the condition for lattice matching with InP, minute regions with different compositions are formed in the crystal, which hinder the movement of electrons, and this is thought to be one of the factors that reduce the electron mobility.

[0054] Increasing the growth temperature and V / III ratio are effective ways to reduce the effect of the miscibility gap. However, it is difficult to increase the growth temperature of GaAsSb because V group elements are prone to desorption from the surface. In addition, it is difficult to increase the V / III ratio during the growth of GaAsSb because it contains Sb, which is prone to surface segregation. Therefore, as long as an Sb molar composition ratio close to the conditions for lattice matching with InP is used, it is difficult to suppress the effect of compositional segregation and improve the electron mobility in GaAsSb.

[0055] In addition to compositional separation, alloy scattering also has an effect on the low electron mobility of GaAsSb. When the Sb molar composition of GaAsSb is y, alloy scattering is proportional to y×(1-y), and the larger this value is, the easier it is for the electron mobility to decrease. Here, the molar composition at which y×(1-y) is maximized is when y=0.5. As mentioned above, GaAsSb lattice-matches with InP when the Sb molar composition ratio (y) is around 0.49, which is close to y=0.5 at which alloy scattering is maximized. Therefore, when GaAsSb with a composition that lattice-matches with InP is used for the base layer, it is considered that the effect of alloy scattering is added to the above-mentioned compositional separation and reduces the electron mobility.

[0056] An effective method for suppressing the compositional separation of GaAsSb mentioned above is to make the Sb molar composition ratio of GaAsSb as far away from 0.5 as possible, as can be seen from Figure 3. This makes it possible to reduce the effect of compositional separation, which is a factor in reducing the electron mobility of GaAsSb. Furthermore, making the Sb molar composition ratio as far away from 0.5 as possible is also effective in reducing the effect of alloy scattering on the electron mobility of GaAsSb.

[0057] In GaAsSb, if we only want to reduce the effects of compositional segregation and alloy scattering, we need only change the Sb molar composition ratio away from 0.5. However, as explained using Figure 17, in a double heterojunction bipolar transistor, it is necessary to reduce the band discontinuity between the base layer and the collector layer, and it is important to determine the Sb molar composition ratio taking this into consideration.

[0058] The Sb molar composition ratio of GaAsSb that is effective for reducing the band discontinuity between the base layer and the collector layer will be described below.

[0059] As mentioned above, there is a large difference between the calculation results and the experimental measurement results of the band alignment of GaAsSb, and it is difficult to calculate the band alignment in the conduction band of GaAsSb and InP with different Sb molar composition ratios. For this reason, in this invention, based on the reported values ​​from known experiments at low temperatures (10K), the band discontinuity of GaAsSb and InP with different Sb molar composition ratios at 300K was calculated using the following method.

[0060] The heterostructure of GaAsSb and InP has a type II band arrangement, as shown in Figure 4. In a structure with this band arrangement, near room temperature (up to 300K), if the potential barrier is small, carriers (electrons, holes) can be thermally excited and overcome this potential barrier. For this reason, when the thickness of GaAsSb is small and carriers photoexcited in GaAsSb immediately reach the heterointerface with InP, emission due to type II interband transition becomes dominant. Even in this case, when the thickness of GaAsSb is large and carriers photoexcited in GaAsSb cannot reach the interface and recombine within GaAsSb, emission due to type I interband transition can also be seen.

[0061] On the other hand, when photoluminescence (PL) is measured at low temperatures, the carriers are less susceptible to thermal excitation, and so light emission due to both type I and type II interband transitions can be observed (see, for example, Non-Patent Documents 3 and 4).

[0062] As can be seen from Figure 4, if the energy due to the interband transition between types I and II is known, the band discontinuity of the conduction band can be found by subtracting the type II emission energy from the band gap of GaAsSb (type I emission energy). The problem is how to reflect the results of photoluminescence measurements at low temperatures in values ​​at room temperature. The inventors used the following method to calculate the band discontinuity of the conduction band at 300K based on the reported values ​​of low-temperature PL.

[0063] To estimate the band discontinuity in the conduction band at room temperature (300K) based on the results of low-temperature measurements, it is necessary to consider the temperature-dependent change in the band gaps of GaAsSb and InP, as well as the temperature-dependent change in the band discontinuity of the conduction band and valence band. g Change in E with temperature T g (T) is the formula of Varshni "E g (T)=E g (T=0)-(αT 2 ) / (T+β)···(1)”.

[0064] In equation (1), T is temperature in Kelvin and E g (T) is the band gap at temperature TK, E g (T=0) is the band gap at temperature 0K, and α and β are constants. In general, α and β change depending on the composition in ternary or more alloy semiconductors, but it is known that GaAsSb is an exception in that they are constant regardless of the Sb molar composition ratio (see Reference 4). Specifically, α = 0.42 meV / K, β = 189 K. For InP, α = 0.363 meV / K, β = 162 K.

[0065] Regarding the temperature-dependent change in the band discontinuity of the conduction band and valence band, in the case of type I quantum well structures, the analysis method is used assuming that the ratio of the band discontinuity between the conduction band and the valence band is constant regardless of temperature. For type II heterostructures, the method of assuming that the ratio of the band discontinuity between the conduction band and the valence band is constant regardless of temperature is also considered to be effective.

[0066] Using the above-mentioned method, type I and type II interband transitions were obtained at room temperature (300K) based on the reported low-temperature PL values. Figure 5 shows the type I and type II interband transitions at 300K obtained based on the experimental results at low temperature (10K) in Non-Patent Document 3. The "x" in Figure 5 indicates a value from an experiment conducted to confirm the usefulness of this analysis method. Specifically, a sample was prepared by growing only GaAsSb with a thickness of 0.3 μm on InP, and the energy of the type I interband transition was obtained from the PL measurement of this sample at 300K.

[0067] The experimental values ​​are in good agreement with the results calculated from the low-temperature experimental data, which indicates that the method used in the present invention is useful for calculating the interband transition energies at room temperature from the low-temperature data.

[0068] If the energies of the interband transitions between types I and II are known, the band discontinuity in the conduction band can be calculated. Figure 6 shows the change in the band discontinuity in the conduction band of a GaAsSb / InP heterostructure at 300 K depending on the Sb molar composition ratio of GaAsSb, calculated from Figure 5. The dotted line in Figure 6 is a straight line approximation of the data points using the least squares method. Figure 6 shows that the data points are almost all along this approximate straight line. This approximation formula is expressed as ΔEc=0.584x-0.169 (eV), where x is the Sb molar composition ratio of GaAsSb and ΔEc is the band discontinuity in the conduction band.

[0069] The band discontinuity in the conduction band between GaAsSb (Sb molar composition ratio: approximately 0.49) that lattice matches InP and InP is approximately 0.12 eV according to the approximation. Therefore, as explained with reference to Figure 17, when GaAsSb that lattice matches InP is used as the base layer and InP is used as the collector layer, a potential notch structure is formed at the bottom of the conduction band of the collector layer, and in this case the band discontinuity in the conduction band is approximately 0.12 eV.

[0070] As can be seen from Figure 6, when the Sb molar composition ratio of GaAsSb is 0.4, the band discontinuity in the conduction band is about 0.06 eV, which is almost half that of GaAsSb lattice-matched to InP. Therefore, if the Sb molar composition ratio of GaAsSb is set to 0.4 or less, the effect of the above-mentioned potential notch structure can be reduced.

[0071] The band discontinuity in the conduction band decreases as the Sb mole fraction of GaAsSb is further reduced, but when the Sb mole fraction is smaller than 0.3, the sign changes from positive to negative. This means that the bottom energy level of the conduction band in the GaAsSb base layer becomes lower than the bottom energy level of the conduction band in the InP collector layer. In this case, a potential barrier in the conduction band is formed between the base layer and the collector layer, which becomes an obstacle to electron movement and causes degradation of device characteristics.

[0072] From the above, when the base layer is made of GaAsSb and the collector layer is made of InP, the Sb molar composition ratio of GaAsSb constituting the base layer is preferably 0.3 or more and 0.4 or less.

[0073] As can be seen from Figure 6, when the base layer is made of GaAsSb and the collector layer is made of InP, a potential barrier against electron migration occurs when the Sb molar composition ratio of GaAsSb is 0.3 or more. As mentioned above, this potential barrier can be reduced by changing the emitter layer from InP to a material with a large band gap, such as InGaP, InAlP, or InAlAs. Of InGaP, InAlP, and InAlAs, InGaP is a material that does not contain Al, which is easily oxidized, and is therefore considered to be useful from the standpoint of reliability.

[0074] The following describes the results of investigating the band discontinuity of the conduction band when the emitter layer is changed from InP to InGaP. 0.8 Ga 0.2This shows the change in the conduction band discontinuity between the emitter layer made of P and the base layer made of GaAsSb depending on the Sb molar composition ratio of GaAsSb. The dotted line in Fig. 7 is a straight line approximation of the data points using the least squares method. The approximation line in Fig. 7 is basically a line obtained by translating the approximation line in Fig. 6.

[0075] As can be seen in Fig. 7, the GaAsSb / In 0.8 Ga 0.2 In the P heterostructure, the band alignment changes from type II to type I when the Sb mole fraction is about 0.36. This is because the emitter layer is In 0.8 Ga 0.2 This means that when the emitter layer is made of P and the base layer is made of GaAsSb, if the Sb molar composition ratio of GaAsSb is 0.36 or less at the interface between the emitter layer and the base layer, no potential barrier against electron movement is generated.

[0076] Of course, the Ga molar composition ratio of InGaP is not limited to 0.2. Figure 8 shows the band discontinuity of the conduction band between InGaP and GaAsSb when the Ga molar composition ratio of InGaP is changed to 0, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30. By increasing the Ga molar composition ratio of InGaP, it is possible to increase the Sb molar composition ratio of GaAsSb at which the above-mentioned band arrangement changes from type II to type I. Specifically, the Sb molar composition ratio is about 0.39 when the Ga molar composition ratio of InGaP is 0.25, and about 0.42 when the Ga molar composition ratio of InGaP is 0.30.

[0077] In this way, by using InGaP for the emitter layer and increasing its Ga molar composition ratio, it is possible to increase the Sb molar composition ratio in the GaAsSb base layer, which reduces the potential barrier against electron movement between the emitter layer and the base layer.

[0078] However, when InGaP is grown on InP, the InGaP crystal lattice is distorted, so there are limitations on the Ga molar composition ratio and thickness. For GaAsSb, the lattice-matching Sb molar composition ratio is about 0.49, and if the Sb molar composition ratio is reduced below this, lattice distortion is added to the crystal lattice.

[0079] Below, we will discuss the lattice strain that actually occurs in InGaP and GaAsSb, as well as methods for reducing the effects of this lattice strain in device structures.

[0080] Figure 9 shows the change in lattice strain with Ga molar composition ratio for InGaP on InP. On the vertical axis of Figure 9, a positive sign indicates compressive strain, and a negative sign indicates tensile strain. As can be seen from Figure 9, as the Ga molar composition ratio increases, tensile strain is applied to InGaP. Specifically, when the Ga molar composition ratio is 0.1, a tensile strain of about 0.7% is applied to the crystal lattice, and when the Ga molar composition ratio is 0.2, a tensile strain of about 1.4% is applied to the crystal lattice. It is known that InGaP on InP can grow without lattice relaxation up to a Ga molar composition ratio of about 0.25 (tensile strain: about 1.8%) (see Reference 5).

[0081] Therefore, if the Ga molar composition ratio of InGaP is 0.25 or less, it can be grown on InP. As mentioned above, it is known that if the band discontinuity in the conduction band is about 100 meV, electron movement is less affected by the band discontinuity in the conduction band. When the Ga molar composition ratio of InGaP is 0.25, the condition for the band discontinuity in the conduction band to be 100 meV or less in Figure 8 is when the Sb molar composition ratio of GaAsSb is 0.53 or less. For this reason, it is desirable for the Sb molar composition ratio of the GaAsSb base layer to be 0.53 or less.

[0082] The change in lattice distortion as a function of the Sb molar composition ratio for GaAsSb on InP is shown in Figure 10. As mentioned above, when the base layer is made of GaAsSb, the condition for maintaining the type II band alignment between the InP collector layer and the base layer while suppressing the effects of compositional separation and alloy scattering is that the Sb molar composition ratio of GaAsSb is in the range of 0.3 to 0.4.

[0083] 10, it can be seen that when the Sb molar composition ratio of GaAsSb is in the range of 0.3 to 0.4, a tensile strain of 0.7% to 1.4% is applied to the crystal lattice of GaAsSb. Therefore, if the base layer is made of GaAsSb with a composition in which the Sb molar composition ratio is 0.3 to 0.4 and the thickness is increased, tensile strain is applied to the entire layer structure, causing lattice relaxation and generating crystal defects.

[0084] However, in double heterojunction bipolar transistors, the band discontinuity of the conduction band is basically a problem at the interface between the emitter layer and the base layer and the interface between the base layer and the collector layer, and it is sufficient if these interfaces have the desired band arrangement. Specifically, the Sb molar composition ratio of the GaAsSb base layer is configured to continuously increase toward the emitter layer. The Ga molar composition ratio of the InGaP emitter layer is also configured to continuously increase toward the base layer.

[0085] Figure 11 shows the change in lattice strain within the layers for structures in which the Sb molar composition ratio in the GaAsSb base layer and the Ga molar composition ratio in the InGaP emitter layer are changed in this way. Figure 11(a) shows the case where the emitter layer is composed only of InGaP. Figure 11(b) shows the case where an InGaP layer is disposed as part of the emitter layer. In both cases, the Ga molar composition ratio in the emitter layer is increased so as to be maximum near the interface with the base layer.

[0086] In either case, the thickness of the InGaP emitter layer can be adjusted by changing the rate of increase of the Ga mole composition ratio. Therefore, the thickness of the InGaP emitter layer can be reduced by increasing the rate of increase of the Ga mole composition ratio. Therefore, the effect of lattice distortion in the InGaP emitter layer can be reduced relatively easily. On the other hand, the thickness of the GaAsSb base layer cannot be reduced easily. This is because the base resistance increases as the thickness of the GaAsSb base layer decreases.

[0087] In the structure of Figure 11, the region where the Sb molar composition ratio of the GaAsSb base layer is small and the tensile strain is large is the layer close to the collector layer. The Sb molar composition ratio of the GaAsSb base layer is constant in the region close to the collector layer, but increases from the middle toward the emitter layer, so the tensile strain is small in this region. Therefore, the structure of Figure 11 is effective in reducing the tensile strain applied to the entire GaAsSb base layer.

[0088] The tensile strain applied to the entire base layer will be explained below. In the GaAsSb base layer shown in FIG. 11, if the thickness of the region close to the emitter layer where the Sb molar composition ratio changes continuously is t1, the absolute value of the average value of the tensile strain is ε1, the thickness of the region close to the collector layer where the Sb molar composition ratio is constant is t2, and the absolute value of the tensile strain is ε2, then the average value of the tensile strain in the entire GaAsSb base layer, ε * is "ε * =(ε1×t1+ε2×t2) / (t1+t2)···(2)".

[0089] In the GaAsSb base layer, the location (region) where the tensile strain is large is the region close to the collector layer, and as mentioned above, the absolute value of the tensile strain ε2 is between 0.7% and 1.4%. Since the Sb molar composition ratio of the GaAsSb base layer increases toward the emitter layer, the tensile strain ε1 in this region is smaller than ε2. Therefore, the average value of the tensile strain ε * can be made smaller than ε2 and can be suppressed to 1% or less.

[0090] For example, in a GaAsSb base layer, the Sb molar composition ratio in the region close to the emitter layer is first set to 0.52. The Sb molar composition ratio is then continuously decreased to 0.3 over a thickness of 15 nm (corresponding to t1) toward the collector layer. After this, the layer is grown to a thickness of 15 nm (corresponding to t2) while keeping the Sb molar composition ratio at 0.3. In this structure, ε1 = 0.6% and ε2 = 1.4%. In this case, the average tensile strain ε * This gives us 1.0%.

[0091] Thus, the average tensile strain ε * can be adjusted by the Sb mole composition ratio and thickness of the GaAsSb base layer shown in Figure 11. However, even in this case, if the thickness of the base layer as a whole becomes too large, the effect of lattice distortion increases and crystal defects occur. In other words, there is an upper limit to the allowable thickness of the base layer as a whole.

[0092] To investigate the upper limit of the allowable thickness of the entire base layer, we fabricated samples in which GaAsSb was grown on InP with a tensile strain of 1% (Sb molar composition ratio 0.36) and the thickness was varied, and X-ray diffraction patterns and microscopic PL mapping measurements were performed for the samples. The samples were grown by metalorganic molecular beam epitaxy, and 3-nm-thick InP was grown on the surface of GaAsSb to suppress oxidation.

[0093] The results of measuring the X-ray diffraction pattern mentioned above are shown in Figure 12. The peak near the incident angle of 32.3 degrees is due to X-ray diffraction from the GaAsSb layer, and it can be seen that the angle of this peak is almost constant regardless of the thickness. This means that in GaAsSb, no significant lattice relaxation occurs even when the tensile strain is 1%.

[0094] Figure 13 shows the results of microscopic PL mapping measurements on GaAsSb samples grown with a tensile strain of 1% and thicknesses of 35 nm and 46 nm. In Figure 13, the higher the concentration in the region, the lower the PL emission intensity. In the case of the GaAsSb sample with a thickness of 35 nm, no dark lines or dark spots with low PL emission intensity were observed. Figure 13 shows an example with a measurement range of 100 μm × 100 μm, but no dark lines or dark spots were observed even when measurements were taken over a wide range of the sample.

[0095] On the other hand, in the case of a sample with a thickness of 46 nm, when an area of ​​several micrometers square was measured, a dark line was observed in only one place, as shown in Figure 13. Such a dark line is often observed when lattice relaxation occurs and crystal defects are generated.

[0096] From the above, it can be seen that when GaAsSb with a tensile strain of 1% is used, it can be grown without generating crystal defects up to a thickness of 35 nm. As mentioned above, in a structure in which the Sb molar composition ratio is changed in the GaAsSb base layer as shown in Figure 11, it is easy to make the average value of tensile strain applied to GaAsSb 1% or less. Therefore, if the overall thickness of the GaAsSb base layer is set to 35 nm or less, it becomes easy to grow a layer structure for devices without generating crystal defects.

[0097] Next, the advantage of using the structure shown in Fig. 11 as the GaAsSb base layer in improving device characteristics will be explained. In the structure shown in Fig. 11, the Sb molar composition ratio of the GaAsSb base layer is a constant value between 0.3 and 0.4 in the region close to the collector layer, but it increases continuously from the middle of the base layer to a value of 0.53 or less toward the interface with the emitter layer. The higher the Sb molar composition ratio of GaAsSb, the higher the energy position of the bottom of the conduction band. Therefore, when a GaAsSb base layer having the structure shown in Fig. 11 is used, the energy position of the bottom of the conduction band will be as shown in Fig. 14. In this case, a pseudo electric field is generated in the base layer close to the emitter layer.

[0098] When electrons enter the base layer from the emitter layer, they are accelerated by this pseudo electric field. However, in this region, the Sb molar composition ratio is large, and electron mobility is low due to the effects of composition separation and alloy scattering, so there is a limit to the acceleration of electrons by the pseudo electric field. On the other hand, in the structure shown in Figure 11, a GaAsSb layer with a small Sb molar composition ratio, from which high electron mobility is expected, is placed in the region close to the collector layer. Furthermore, the Sb molar composition ratio in this region is constant, and the tensile strain does not increase as it approaches the collector layer, as in the region where a pseudo electric field is applied, and the tensile strain is also constant.

[0099] As a result, the GaAsSb base layer can accelerate electrons while suppressing the occurrence of crystal defects due to lattice distortion, and can utilize high electron mobility. In this structure, the GaAsSb base layer has a degree of design freedom not found in structures using conventional GaAsSb base layers, such as the thickness of the region where the Sb molar composition ratio is constant and the region where it changes continuously, and the rate of increase in the Sb molar composition ratio in the region where it changes continuously. By setting these values ​​appropriately, the current gain cutoff frequency can be increased.

[0100] As described above, it is understood that the layer structure of the double heterojunction bipolar transistor according to the embodiment can improve the device characteristics. The above explanation shows an example in which the Sb molar composition ratio of the GaAsSb base layer is constant in the region close to the collector layer, but it does not necessarily have to be constant as long as the average value of the tensile strain applied to GaAsSb is 1% or less and the overall thickness of the GaAsSb base layer is 35 nm or less. A structure in which the Sb molar composition ratio of the GaAsSb base layer in the region where the Sb molar composition ratio is constant gradually decreases toward the collector layer is also effective. This is because the effect of the small influence on the miscibility gap and the electron mobility due to the alloy is utilized even if the Sb molar composition ratio is reduced in the range where the Sb molar composition ratio of the GaAsSb base layer is small and the influence of the lattice strain is small.

[0101] In the above-described embodiment, the base layer is made of only GaAsSb. However, as long as the average value of the tensile strain applied to the base layer is 1% or less and the overall thickness of the GaAsSb base layer is 35 nm or less, the base layer does not necessarily have to be made of only GaAsSb. Needless to say, it is effective for the base layer to contain a small amount of In within a range that does not significantly affect the magnitude of the tensile strain and the electron mobility.

[0102] As described above, according to the present invention, the Sb molar composition ratio of the base layer made of a III-V compound semiconductor containing Ga, As, and Sb decreases in the thickness direction from the emitter layer side to the middle of the base layer and is constant from the middle of the base layer to the collector layer. Therefore, a heterojunction bipolar transistor using GaAsSb for the base layer can obtain expected device characteristics.

[0103] It should be noted that the present invention is not limited to the above-described embodiments, and it is apparent that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0104] [References] [Reference 1] TH Chiu et al., "Molecular beam epitaxy of GaSb0.5As0.5 and AlxGa1-xSbyAs1-y lattice matched to InP", Applied Physics Letters, vol. 46, no. 4, pp. 408-410, 1985. [Reference 2] K. Miura et al., "The growth of high quality GaAsSb and type-II InGaAs / GaAsSb superlattice structure", Journal of Applied Physics, vol. 113, 143506, 2013. [Reference 3] VS Sorokin et al., "Novel approach to the calculation of instability regions in GaInAsSb alloys", Journal of Crystal Growth, vol. 216, pp. 97-103, 2000. [Reference 4] R. Lukic-Zrnic et al., "Temperature dependence of the band gap of GaAsSb epilayers", Journal of Applied Physics, vol. 92, no. 11, pp. 6939-6941, 2002. [Reference 5] M. Kahn, and D. Ritter, "Strain relief by long line defects in tensile GayIn1-yP layers grown on InP substrates", Applied Physics Letters, vol. 79, no. 18, pp. 2028-2930, 2001. [Explanation of symbols]

[0105] 101...substrate, 102...sub-collector layer, 103...collector layer, 104...base layer, 104a...first base layer, 104b...second base layer, 105...emitter layer, 105a...InGaP layer, 105b...upper emitter layer, 106...emitter cap layer, 111...collector electrode, 112...base electrode, 113...emitter electrode.

Claims

1. A substrate made of InP; a collector layer formed on the substrate and made of a III-V compound semiconductor; a base layer formed on the collector layer and made of GaAsSb; an emitter layer formed on the base layer and made of a III-V compound semiconductor different from the base layer; Equipped with The Sb molar composition ratio of the base layer decreases in the thickness direction from the emitter layer side to the middle of the base layer, and is constant from the middle of the base layer to the collector layer. A heterojunction bipolar transistor comprising:

2. 2. The heterojunction bipolar transistor of claim 1, the Sb molar composition ratio of the base layer is in the range of 0.49 to 0.53 near the interface with the emitter layer in the thickness direction, and in the range of 0.3 to 0.4 near the interface with the collector layer, The thickness of the base layer is set to 35 nm or less. A heterojunction bipolar transistor comprising:

3. 3. The heterojunction bipolar transistor according to claim 1, the emitter layer includes an InGaP layer made of InGaP in a part of the thickness direction, The Ga mole fraction of the InGaP layer is in the range of more than 0 and not more than 0.25, and increases toward the base layer. A heterojunction bipolar transistor comprising:

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