LOW RESISTANCE CONTACT SEMICONDUCTOR DIODE

Abstract: A semiconductor device wherein the layer of highly doped p-type material (2) typically found in devices of the prior art is replaced with a layer of highly doped n-type material (11) and a thin layer of highly doped p-type material (2) thus facilitating low resistance contact, transparency to radiation produced by the device and confinement with low loss of radiation produced by laser devices.

Description: Low Resistance Contact Semiconductor Diode. The current invention relates to devices made from narrow-gap (and more conventional) semiconductors for operation at infrared wavelengths. Many such devices include a diode junction, hence metal contacts need to be formed to a region of n-type and a region of p-type material. Ideally the metal contact should be ohmic and have a low resistance, i.e. very much less than that of the junction. The surfaces ofthe semiconductors tend to comprise a native oxide, which is normally insulating to some extent and must be removed or disrupted to permit contact to the semiconductor underneath. The nature ofthe band-bending which often occurs at the metal-semiconductor interface means that it is relatively easy to obtain the desired contact to n-type material, but it can be difficult to achieve an ohmic. low resistance contact to the p-type semiconductor. Alloying using a metal with a small proportion of a like-type dopant can be used with some semiconductors. When annealed, this gives a heavily doped region to which the metal is more likely to form an ohmic contact, for example Au/Zn for a p + p contact to GaAs. This does not work well, however, with the narrow-gap semiconductors, such as indium antimonide (InSb) and cadmium mercury telluride (CMT) because of their low melting temperatures and the high diffusion rates of some of the dopants. Ion milling is often used for the narrow-gap materials to remove the surface oxide. This tends to result in near surface (typically tens of nanometres) damage which is heavily n-type in nature plus additional damage which can propagate significantly further into the material and degrade the junction properties. As a consequence the thickness of the contact regions in diodes made from the narrow-gap semiconductors needs to be greater than, or of the order of, 10 " m. In devices such as light emitting diodes and diode lasers where optical radiation passes through one or both contact regions, this thickness of material can lead to significant optical losses which will degrade the device performance, in some cases catastrophically. Degenerate doping ofthe material, i.e. doping to a sufficiently high level that the Fermi energy lies within the appropriate band, leads to a Moss-Burstein shift whereby the absoφtion coefficient for radiation of energy just greater than the band-gap decreases so that it becomes transparent. Owing to the band structure ofthe narrow-gap semiconductors, however, it is extremely difficult to induce degenerate doping levels in p-type material, so that a sufficient Moss-Burstein shift is not observable. In contrast, it is comparatively easy to produce degenerate doping in n-type material and large Moss-Burstein shifts can be achieved. Hence the n-type contact regions of diodes can be made transparent to radiation generated or absorbed in the active region but the p-type contact region can not. According to the current invention. a semiconductor device comprises an active layer of p- type or n-type material forming a junction with a first layer of highly doped n-type material, a second layer of highly doped n-type material adjacent to a layer of highly doped p-type material, which may be adjacent to, or separated from by other layers, the active layer of p- type or n-type material, and means for providing electrical contact with the device including means for providing electrical contact, via the second layer of highly doped n-type material, with the adjacent layer of highly doped p-type material. The invention should not be regarded as being restricted to a particular set of doping levels. The term "'highly doped" when applied to layers within devices ofthe current invention should be construed as a relative term; thus layers to which the term is applied are understood to be more highly doped than those to which it is not. Preferably the semiconductor band-gap ofthe active layer is of less energy than 0.5eV and one or both ofthe first and second layers of highly doped n-type material is transparent to radiation, of energy greater than that ofthe band-gap. which is emitted or absorbed by the device. In a specific embodiment the device comprises a front surface emitting positive LED. negative LED or detector. J In another specific embodiment the device comprises a back surface emitting positive LED, negative LED or detector wherein the means for providing electrical contact, via the second layer of highly doped n-type material, with the adjacent layer of highly doped p-type material comprises a metallic contact and the second layer of highly doped n-type material provides a transparent front contact region to facilitate the use ofthe metallic contact as a mirror. In another specific embodiment the device comprises a laser diode and the first and second layers of highly doped n-type material provide optical confinement within the active layer. In another specific embodiment the second layer of highly doped n-type material provides an electrical path to an excluding or extracting contact. In this embodiment the device may comprise a field effect transistor or a bipolar transistor. The invention will now be described, by way of example only, with reference to the following figures in which: figure la and lb respectively show generic devices ofthe prior art and ofthe invention; figure 2a and 2b respectively show the band diagrams, at zero bias, of the devices shown in figures la and lb; figures 3a and 3b respectively show light emitting diodes ofthe prior art and the current invention; figures 4a and 4b respectively show back surface emitting devices of the prior art and the current invention; figures 5a and 5b respectively show semiconductor laser devices of the prior art and the current invention; figures 6a - 6c show various stages of construction of a semiconductor laser device according to the current invention and figure 7 shows the performance ofthe device of figure 6c. Throughout this specification p and n indicate the type of doping in a material, the superscripts - and + indicate light and heavy doping respectively, the underline indicates material of high band gap, v denotes an active layer of near intrinsic n-type material and π denotes an active layer of near intrinsic p-type material. The invention is described in relation to light emitting diodes and lasers fabricated in the indium aluminium antimonide (In,. x Al x Sb) material system, but is not limited to this system. The generic concept is applicable to both types of device, and will be described first, but its detailed design criteria and implementation are slightly different for each case and these will be described subsequently. Example 1 : Generic concept. By the current invention the majority of the highly doped p-type contact region immediately adjacent to the contact in a diode ofthe prior art is replaced with highly doped, degenerate n-type material, leaving only a thin (ofthe order of 0.1 x 10 "6 m thick) highly doped (but not necessarily degenerate) p-type region adjacent to the remainder ofthe diode. The thickness ofthe remaining p region is sufficiently small that it does not result in significant absorption ofthe optical radiation. The electrical junction formed between these n + and p + regions will be biased in the opposite sense to the active diode so, for example, if the active diode was forward biased the 'contact' diode would be reverse biased and might be expected to introduce a large series resistance. It is a property of narrow-gap semiconductors, however, that when heavily doped regions of opposite carrier type are in immediate proximity to each other, the probability of carrier tunnelling between the regions is very high even with reduced potential across the junction. Hence a very large tunnelling current can pass through the n + p + diode and it does not present a significant series resistance. Therefore the requirement for transparent contact regions on both sides ofthe active region can be fulfilled and efficient device operation can be achieved. Referring to figure la, a typical 4-layer structure 1, of the prior art comprises a layer of p + material 2, a layer of p + material 3 such as In,. x Al x Sb, an active layer of π-type material 4 and a layer of n + material 5. Electrical contacts 6 are made with layers 2 and 5. Figure 2a qualitatively shows the relative energies ofthe valence band 7, conduction band 8 and Fermi level 9 of the device shown in figure la. The design criteria and operation of this structure in the context of LEDs, detectors and transistors has been published previously (see T. Ashley, CT. Elliott. N.T. Gordon. R.S. Hall, A.D. Johnson and G.J. Pryce, 'Uncooled InSb In^Al^Sb mid-infrared emitter', App. Phys. Lett. 64 2433 (1994); T. Ashley, A.B. Dean, CT. Elliott, G.J. Pryce, A.D. Johnson and H. Willis, 'Uncooled high-speed InSb field-effect transistors', Appl. Phys. Lett. 66 481 (1995) and T. Ashley, 'Electronic and optoelectronic devices in narrow-gap semiconductors', Proc. 7th Int. conf. Narrow-Gap Semiconductors, Santa Fe, NM, USA 9- 12 Jan 1995. A device of the current invention 10 (comprising a modified, tunnel contact 5-layer structure) and its band diagram are shown in figures lb and 2b respectively. In all the figures of this specification, corresponding items performing a similar role are labelled with the same numeral. In addition to the first layer of n material 5, performing a role similar to that of layer 5 in figure la, device 10 has a second layer of n material 1 1 through which electrical contact with p + layer 2 is made. The thickness of p + layer 2 is greatly reduced in comparison with that of layer 2 in device 1. The region ofthe semiconductor where tunnelling occurs is shown schematically by region 12 in figure 2b. In the case ofthe device shown, the active region is π-type but this is not essential to the invention and it could have a different doping type and level (for example v type) in other devices, and could even be a multiplicity of layers. Table 1 lists typical thicknesses for the layers of devices 1 and 10, but the invention is not generally limited to these values. Table 1: Layer thicknesses for devices 1 and 10 shown in figures la and 2a respectively. Thickness (m x 10 " °) Layer Device 1 Device 10 1 1 - 0.9 2 1 0.1 3 0.02 0.02 4 3 3 5 1 1 Calculation ofthe tunnelling probability indicates that in InSb at room temperature (294K) with a doping of 1 xlO cm " on each side ofthe n + /p + junction that the maximum ofthe product of area and resistance in reverse bias should be less than 10 " Ω cm . This is about half the value for a comparable device of the prior art. The tunnel junction effectively forms a large recombination velocity contact to the p region, which could potentially inject additional electrons over the In x Al Sb barrier (layer 3) in to the active diode and degrade its performance. Modelling indicates that for a diode at 294K with a barrier of composition x=0.15 and a p + doping of 3 x 10 18 cm " \ the p + thickness can be reduced to 0.01 x 10 " m without decreasing the active diode R < ,A (product of resistance at zero bias and area) by more than 1%. The value of 0.01 x 10 "6 m represents a lower figure for the accuracy of the model rather than a predicted minimum thickness. The maximum thickness ofthe p * region 2 is determined by the acceptable optical loss or electrical series resistance, as discussed below for the specific devices. If a thickness of more than 10 " m is acceptable, it is not necessary to use the n + tunnel contact simply to avoid damage to the active diode as the p region alone will suffice to absorb any contacting damage. However the n + region 11 also has the benefit of a very much lower (approx. factor of 50 per unit thickness) resistance, which can be very important if the lateral resistance of a contact region is significant. Example 2; Light Emitting Diodes. Included within this group of devices are positive luminescence diodes, negative luminescence diodes detectors and vertical cavity surface emitting lasers. In all of these devices, light is transmitted essentially vertically through one of the contact regions ofthe diode. 1. Front surface devices: Referring to figure 3a. a diode 1 ofthe prior art where the radiation, absorbed or emitted in active region 4, is to be transmitted through the front or top surface, would normally have an n + top contact region 5, doped sufficiently highly that it is transparent, and the p ^ contact region 2 would be at the bottom of the structure adjacent to the substrate not shown. This can lead to a large lateral resistance (R p ) along the p-type material 2 which is comparable to the diode \ . This would lead to extra power dissipation in LEDs and would degrade the signal to noise performance of a detector. Referring to figure 3b, the use of a parallel n region 1 1 with resistance R n and a tunnel junction with resistance R t such that R n + R, « R p would reduce the series resistance and so reduce power dissipation in the LEDs and improve signal to noise ratio in the detectors. Back surface devices. Referring to figure 4a, radiation, absorbed or emitted in active region 4, is intended to be transmitted through the substrate of prior art device 1 and the back surface would normally be designed with the p + region 2 uppermost, so that the light passes through a transparent n + region 5 and a transparent n + substrate (not shown). In this case the top metal contact 6 can be placed over the whole ofthe upper surface to form a mirror to reflect light back through the active region 4. A single thick p + layer 2 attenuates this reflected light, negating the benefit ofthe mirror. Referring to figure 4b the use of an n + region 1 1 to form a tunnelling junction to thin p + material 2 enables it to be transparent so that a higher proportion of the light is reflected. Example 3: Lasers. Referring to figure 5 a, a typical semiconductor laser 13 requires an active gain region 4 surrounded by cladding layers 2, 5 which provide electrical confinement ofthe carriers and optical confinement and guiding ofthe emission. The prior art structure described above in the context of LEDs and detectors provides excellent electrical confinement, however it does not give good enough optical confinement. Optical confinement is obtained through a change (decrease) in refractive index at the interfaces with the cladding regions which guides the radiation back into the active region. A reduction in the refractive index can be achieved in the InSb material system through high doping, however only highly doped n- type layer 5 also becomes transparent to radiation just above the band-gap: the highly doped p-type layer 2 remains absorbing. Hence, even though some guiding is achieved by the p cladding layer 2 it strongly attenuates a wave propagating down the laser leading to the requirement for a very much higher gain in the material before stimulated emission is obtained - effectively preventing the laser from operating. Referring to figure 5b, a laser device 14 ofthe current invention includes an n + layer 1 1 which forms a tunnel junction with p + layer 2 and permits guiding with low modal loss to be obtained on both sides ofthe active gain region 4 thus enabling the laser to operate. Devices 13 and 14 would typically be fabricated on an n + substrate 15. Table 2 lists typical thicknesses for the layers of devices 13 and 14, but the invention is not generally limited to these values. Table 2: Layer thicknesses for devices 13 and 14 shown in figures 5a and 5b respectively. Thickness (m x 10 ' °) Laver Device 13 Device 14 11 - 1 2 1 0.1 3 0.02 0.02 4 3 3 5 3 3 In order to produce an actual embodiment ofthe laser described generically in figure 5b layer structures as shown in therein were grown on highly doped n-type InSb substrates 15 doped within the range 10 cm to 3 x 10 cm " . The layer thicknesses were as indicated in table 2. Referring to figure 6a, the substrates were generally circular with a diameter of 1.5 or 2 inch, and were sawn or cleaved into chips 16 of approximately 6mm square after growth. The thickness ofthe chips was reduced, by polishing from the substrate side, from an initial thickness of 5 x 10 m to 1.5 - 2 x 10 m. Conventional photolithographic means were used to etch down to the lower epitaxial n + layer (item 5 of figure 5b) to form mesa structures 17 of width 50 x 10 "6 m or 100 x 10 " m on a pitch of approximately 1mm across the chip. Referring to figure 6b, similar photolithographic means were used to deposit chromium/gold metallic contacts 6 on to the top of each mesa 17 and. in some cases, to the common lower n + layer (item 5 of figure 5b). Each chip was then further cleaved, in a direction peφendicular to the mesa length, into pieces approximately 5 x 10 m wide to form bars 18 each forming individual lasers which emitted radiation via the facets on the long sides of the bars as represented by arrows 19 in figure 6c. Each bar was mounted separately on a standard sample package and gold wire bonds were applied to the top contact of each laser and to the common contact for subsequent assessment and use. Modelling indicates that for the structure 14 shown in figure 5b, with the dimensions indicated in table 2, the absoφtion coefficient along the laser is only -0.0037/m x IO '6 , compared with -0.06/m x 10 " in the structure 13 of figure 5a. Fabrication of this structure has allowed the first demonstration of an electrically pumped laser in InSb at a temperature of 77K, see the output spectrum in figure 7. Previously, devices required cooling to substantially lower temperatures and/or the use of high magnetic fields. Example 4 Transistors. The use of exclusion/extraction to enable room temperature operation of InSb based transistors has been patented (GB 2 266 183). Several types of these transistors, in particular the field effect devices, require the use of a p + region to form an excluding contact, which carries a significant current in a lateral direction. The use of an n + tunnel junction would reduce the voltage loss along this region in a similar manner to that described for the front surface LEDs.

Claims: Claims 1. A semiconductor device comprising an active layer of p-type or n-type material forming a junction with a first layer of highly doped n-type material, a second layer of highly doped n- type material adjacent to a layer of highly doped p-type material, which may be adjacent to, or separated from by other layers, the active layer of p-type or n-type material, and means for providing electrical contact with the device including means for providing electrical contact, via the second layer of highly doped n-type material, with the adjacent layer of highly doped p-type material. 2. The device of claim 1 wherein the semiconductor energy band-gap of the active layer is less than 0.5eV. 3. The device of claim 1 or 2 wherein the second layer of highly doped n-type material is transparent to radiation, of energy greater than that of the band-gap, which is emitted or absorbed by the device. 4. The device of claim 1 or 2 wherein said device comprises a front surface emitting positive LED. negative LED or detector. 5. The device of claim 3 wherein said device comprises a back surface emitting positive LED, negative LED or detector wherein the means for providing electrical contact, via the second layer of highly doped n-type material, with the adjacent layer of highly doped p-type material comprises a metallic contact and the second layer of highly doped n-type material provides a transparent front contact region to facilitate the use of the metallic contact as a mirror. 6. The device of claim 3 wherein said device comprises a laser diode and the first and second layers highly doped of n-type material provide optical confinement within the active layer. 7. The device of claim 2, wherein the second layer of highly doped n-type material provides an electrical path to an excluding or extracting contact. 8. The device of claim 7 wherein said device comprises a field effect transistor. 9. The device of claim 7 wherein said device comprises a bipolar transistor.

Inventors: ASHLEY TIMOTHY, PRYCE GRAHAM JOHN, ASHLEY, TIMOTHY, PRYCE, GRAHAM, JOHN
Applicants: ASHLEY TIMOTHY, PRYCE GRAHAM JOHN, SECR DEFENCE, ASHLEY, TIMOTHY, PRYCE, GRAHAM, JOHN, THE SECRETARY OF STATE FOR DEFENCE
Application year: 1996
Main classification: H01L33
IPC codes: H01L21, H01L29, H01L31, H01L33, H01S5