Optoelectronic device designed as a VCSEL with a heat-spreading layer and method for manufacturing such a device

Abstract

Optoelectronic device designed as a VCSEL with a p-doped, upper, inner DBR grid (14), an active layer (24) for generating radiation with a predetermined wavelength, a heat spreading layer (26), an n-doped, lower, inner DBR grid (32) and a p-doped, outer DBR grid (34) wherein the active zone (24) is arranged between the inner, upper and lower DBR grids (14, 32), wherein the heat spreading layer (26) made of AlAs with a thickness greater than 100 nm is arranged between the active zone (24) and the lower, inner DBR grid (32), wherein the upper DBR grid (14) has an electrically non-conductive aperture (16), wherein the outer DBR grating (34) is designed such that wavelengths below the specified wavelength are filtered or absorbed, wherein the outer DBR grid (34) is also part of a pin diode (40) below it, which also has an absorbing intrinsic layer (36) and an n-layer (38), wherein a metal contact (22) is provided on a top side of the active zone (24), wherein an n-conducting diffusion region (23) extends below the metal contact (22) to at least the heat spreading layer (26) and electrically connects it to the metal contact (22).

Description

[0001] The invention relates to an optoelectronic component designed as a VCSEL and a method for manufacturing an optoelectronic component according to the preamble of the independent claims.

[0002] From EP 1256151 B1, a VCSEL with a monolithically integrated photodetector is already known, with an active layer intended for radiation generation between DBR grids as reflectors and a photodetector provided with a radiation-absorbing layer within one of the DBR grids.

[0003] US patent 2011 / 0064110 A1 discloses an optoelectronic device as a VCSEL with a monolithically integrated photodiode. This photodiode is integrated into the lower DBR grid and consists of a sequence of n-doped, p-doped, intrinsic, and further n-doped layers. A common n-contact for the VCSEL and the photodiode is provided. A spectral filter, a doped quantum film layer, is integrated in the p-doped region of the DBR to block spontaneous emission. The DBR grids are made of AlGaAs, with the aluminum content in the absorbing layers of the photodiode being reduced to optimize absorption, while it is higher in the DBR mirrors.

[0004] To filter short-wavelength spontaneous emission, a specific doped quantum film layer is used within the DBR. The present application differs in that the filtering effect is achieved directly through the design of the p-type outer DBR lattice itself, by reducing the aluminum content in the low-Al layers to generate fundamental absorption, thus eliminating the need for a separate filter layer.

[0005] Furthermore, the solution described in US patent 2011 / 0064110 A1 is designed for wavelengths such as 940 nm, at which AlGaAs is transparent. The approach does not work for red VCSELs (e.g., 680 nm).

[0006] Furthermore, US 2011 / 0064110 A1 does not disclose a heat spreading layer.

[0007] US patent 2005 / 0271113 A1 discloses an optoelectronic device with improved heat dissipation. The core concept involves integrating a heat-spreading layer with high thermal conductivity into a VCSEL. This layer is positioned between the active layer and the lower DBR grid or the upper DBR grid, reducing the temperature and temperature gradients in the active zone. Aluminum arsenide is proposed as a material for the heat-spreading layer. Additionally, an electrically non-conductive aperture can be incorporated into the upper DBR grid to limit current and optimize light emission efficiency.

[0008] While the document reveals a heat-spreading layer, it does not address the challenges of using aluminum arsenide, particularly its reactivity with ambient air and the need to protect it from oxidation. No specific manufacturing process that solves this problem is described.

[0009] German patent DE 10 2011 085077 A1 discloses a surface-emitting semiconductor laser whose DBR mirror has at least two mirror layers with different absorption coefficients, dopant concentrations, or material compositions. The aim is to reduce laser radiation absorption and optimize electrical resistance in order to homogenize the current distribution.

[0010] US patent 2005 / 0041714 A1 discloses a VCSEL with a multi-layered Bragg reflector, the lower reflector incorporating a photodetector for detecting the emitted laser beam. The active layer of the assembly is sandwiched between two cladding layers. Given its intended use at long wavelengths, it can be assumed that the cladding layers are also made of GaAs to ensure transparency at long wavelengths.

[0011] US patent 2002 / 0121647 A1 deals primarily with a modification of a pseudomorphic high electron mobility transistor (PHEMT) structure, whereby such components can be implemented as waveguide or vertical cavity components.

[0012] Coherent optical measurement technology enables fast and accurate distance and velocity measurements. This allows for the use of both 1D and 3D sensors. Various light sources and detectors can be employed. The light source should emit monochromatic light and, depending on the application, its frequency should also be variable. Such light sources are typically used in the near-infrared range. Depending on the optics used, the interference patterns to be detected are quite small. The detector must have a pixel size or pixel pitch small enough to detect the interference pattern without loss of information.

[0013] The invention should take the following points into account: The light source should be visible to the human eye.

[0014] Complicated adjustment of optical components or alignment of the light source and detector should be avoided if possible.

[0015] The detector should be small enough to detect locally fine interference structures and allow a high detection bandwidth of ~1MHz for the detection of moving objects in the range of ~1m / s via the Doppler shift.

[0016] The light source should consume little energy and the emission frequency should be able to be changed in a controlled manner so that the sign of a moving object can also be determined.

[0017] The polarization should be linear and stable.

[0018] A red single-mode VCSEL with stable polarization and integrated photodiode with high output power for the relevant temperature range and good lifetime is an attractive solution for this application.

[0019] The task is to present an optoelectronic component to solve the aforementioned requirements.

[0020] The problem is solved by the device and method according to the invention as defined in the independent claims.

[0021] Advantageously, an optoelectronic device designed as a VCSEL is provided, comprising a p-doped, upper, inner DBR grid, an active layer for generating radiation with a predetermined wavelength, a heat spreading layer, an n-doped, lower, inner DBR grid, and a p-doped, outer DBR grid. the active zone is located between the inner, upper and lower DBR grids, wherein the heat spreading layer made of AlAs with a thickness greater than 100 nm is arranged between the active zone and the lower, inner DBR grid (32), wherein the upper DBR grid has an electrically non-conductive aperture, wherein the outer DBR grating is designed such that wavelengths below the specified wavelength are filtered or absorbed, where the outer DBR grid is also part of a pin diode below it, which also has an absorbing intrinsic layer and an n-layer.

[0022] This approach has the advantage that the waste heat from the active zone can be effectively dissipated.

[0023] They show schematically Fig. 1 an optoelectronic component according to the invention, Fig. 2 a light and power distribution in the component according to Fig. 1.

[0024] VCSELs as light sources with integrated photodiodes in the structure processed as semiconductor chips to enable coherent detection are preferred according to the invention.

[0025] Problem of electrical contacting "intra-cavity": No VCSELs exist that generate laser light at wavelengths below 700 nm and have an integrated photodiode to enable coherent detection. The reason is a heat-spreading layer made of AlAs, which is placed as close as possible to the active zone, where the laser light is amplified, to enable laser operation even at high temperatures. This AlAs must not be etched away to contact the underlying photodiode, as AlAs is highly reactive with its environment. The solution proposed here is the realization of a diffusion contact above the AlAs layer in the largely undoped material system AlGaInP or, if necessary, other material systems.

[0026] Problem of interfering with light at the photodiode: The integrated photodiode should ideally detect no interfering light to improve signal detection. For this purpose, a wavelength filter integrated into the DBR is proposed, which simplifies the problem of thick epitaxial layers and thus the yield-sensitive fabrication of VCSEL devices.

[0027] Fig. 1 and Fig. Figure 2 shows a VCSEL 1 according to the invention with a photodiode. The VCSEL structure 1 is preferably designed for a laser wavelength of less than 700 nm; of course, this structure can also be designed for other wavelengths or other material systems.

[0028] The optoelectronic device comprises, from top to bottom: a p-type contact 21, a p-type inner DBR grid 14 made of AlGaAs, an oxidation aperture 16, n-type contacts 22 on an active layer 24, a heat spreading layer 26 made of AlAs, an n-type inner DBR grid 32, a p-type outer DBR grid 34, an intrinsic i-layer 36, an n-layer 38, an n-doped GaAs substrate 45 and an n-type contact 50.

[0029] The pin photodiode 40, formed from the areas 34, 36 and 38, works with AlGaAs as a material with light-absorbing function, whereby the proportion of aluminum in AlGaAs is optimized to the wavelength (absorption capacity) of the light in order to minimize stray light (=spontaneous light) and thus represents a part of the wavelength-selective filter.

[0030] The actual VCSEL structure contains a p-type DBR 34 (Distributed Bragg Reflector) mirror region with spectral filter properties to minimize stray light reaching the photodiode. For this purpose, the proportion of aluminum in the AlGaAs within the DBR mirror 34 is selected such that stray light is strongly attenuated compared to light in the desired wavelength range. The electrical connections of the reverse-biased pin photodiode 40 are made via a substrate contact 50 on the n-type side and via a forward-biased pn-30 or pin-40 heterojunction on the p-type side 34 of the photodiode. The intra-cavity n-contact 22, 32, 34 is simultaneously the n-contact 22, 32, 34 of the VCSEL pin diode 24, which amplifies the light and enables laser operation, among other applications. The p-contact 12 of the VCSEL structural component is formed via a mesa contact. The resulting n-contact in the overall structure, or...The n-contact 22 of the VCSEL structural component takes place either in the largely undoped semiconductor system AlGaInP or in the adjacent AlAs or AlGaAs layers with diffused metal.

[0031] The heat-dissipating layer 26, made of the highly thermally conductive and highly reactive aluminum arsenide (AlAs), is not directly exposed to the environment and remains unetched. Thus, its heat dissipation capacity is maintained. The thickness of the heat-dissipating layer is preferably greater than 100 nm, and particularly preferably greater than 150 nm.

[0032] Electrical contacts on semiconductors are usually created using a combination of a highly doped semiconductor crystal, produced by epitaxy, and deposited / vapor-deposited metal layers. Both the doping and the metal layers must be treated differently depending on the type of conductivity required: n-type or p-type (electrons or holes) (doping elements, metal elements, and layer thicknesses).

[0033] The active zone has a nominally undoped area to minimize defect concentrations and optimize lifetime properties in this optically amplifying zone. Lateral doping structuring is not possible during the epitaxy process.

[0034] The innovation lies in the use of the undoped layers of the active zone as a common contact layer for the n-contact of the VCSEL and the p-contact of the photodiode, which is converted into an n-conducting contact via the pn-diode in the DBR.

[0035] Since the etched profiles and the electrically non-conductive oxide layer prevent any significant flow of p-type charge carriers (hole conduction) in the laterally outer region, this area is not optically amplifying and can be used for contacting. For this purpose, n-type conductivity must be achieved down to the underlying nominally n-doped layers via diffusion of dopants and sealed with vapor-deposited contact layers. This is shown schematically in Fig. 1 shown.

[0036] Stray light in the photodiode, dominated by spontaneous emission from the active zone in the VCSEL structural component, significantly alters the properties of the photodiode and must be suppressed or filtered as effectively as possible.

[0037] For this purpose, the spectral absorption capacity of AlGaAs is used as a function of the aluminum content.

[0038] To ensure that the photodiode operates with an absorption coefficient of >10000 / cm² (room temperature) at a given wavelength of 690 nm, for example, an aluminum content of 27% of the group III elements (gallium and aluminum) in AlGaAs (Al0.27Ga0.73As) is used. Wavelengths above 690 nm are then detected only with significantly reduced absorption.

[0039] For wavelengths shorter than the target wavelength of 690 nm mentioned in the example, thick absorption layers with a higher aluminum content are typically used, for which the target wavelength of 690 nm and above is largely transparent. In this example, this could be 31% (Al0.31 Ga0.69As). These layers, several micrometers thick, must also be grown by epitaxy.

[0040] The real innovation lies in avoiding this additional layer by using a modified DBR mirror component. In the first section of the DBR mirror, the smallest aluminum component is selected to be largely transparent for the target wavelength but strongly attenuates shorter wavelengths. In this example, with a target wavelength of 690 nm, the aluminum component would then be 31%. With a corresponding total thickness of all layers containing this aluminum component, this results in comparable absorption at these interfering wavelengths.

[0041] Another advantage is the associated refractive index jump, which increases the reflectivity of the mirror while requiring fewer mirror pairs. However, a higher minimum aluminum content is used for the DBR mirror section near the active zone, as residual absorption in the DBR near the optically amplifying active zone is critical.

Claims

[1] Optoelectronic device designed as a VCSEL with a p-doped upper inner DBR grid (14), an active layer (24) for generating radiation with a predetermined wavelength, a heat spreading layer (26), an n-doped lower inner DBR grid (32) and a p-doped outer DBR grid (34) wherein the active zone (24) is arranged between the inner, upper and lower DBR grids (14, 32), wherein the heat spreading layer (26) made of AlAs with a thickness greater than 100 nm is arranged between the active zone (24) and the lower, inner DBR grid (32), wherein the upper DBR grid (14) has an electrically non-conductive aperture (16), wherein the outer DBR grating (34) is designed such that wavelengths below the specified wavelength are filtered or absorbed, wherein the outer DBR grid (34) is also part of a pin diode (40) below it, which also has an absorbing intrinsic layer (36) and an n-layer (38), wherein a metal contact (22) is provided on a top side of the active zone (24), wherein an n-conducting diffusion region (23) extends below the metal contact (22) to at least the heat spreading layer (26) and electrically connects it to the metal contact (22). [2] Optoelectronic device according to claim 1, wherein the number of layers of the inner lower DBR grid (32) is greater than the number of layers of the outer DBR grid (34). [3] Method for manufacturing an optoelectronic component according to any one of the preceding claims, in which the side areas of the upper DBR grid (14) are exposed up to the active layer by an etching process up to the top of the active zone (24), wherein dopants are applied in the area of ​​the intended metal contacts (22), wherein a thermal process is controlled such that the dopants at least reach the heat spreading layer (26) and establish an n-conducting contact, wherein the pin diode (40), the DBR grids (14, 32, 34), the heat spreading layer (26) and the active zone (24) are built on an n-doped GaAs substrate (45), the diffusion area (23) extends deeper into the p-doped DBR area (34) and thus electrical contact of the p-doped DBR area (34) is made via the metal contact (22).

Images