Ge—GaAs heterojunction-based SWIR photodetector

Abstract

Photodetectors comprising a P type Ge region having a first region thickness and a first doping concentration and a N type GaAs region having a second region thickness and a second doping concentration smaller than the first doping concentration by at least one order of magnitude.

Claims

1. A photodetector, comprising: a heterojunction comprising a P type Germanium (Ge) layer having a first layer thickness and a first doping concentration; and a N type Gallium Arsenide (GaAs) layer having a second layer thickness and a second doping concentration smaller than the first doping concentration by at least one order of magnitude, wherein the Ge layer is adjacent to the GaAs layer and wherein the photodetector is a short wave infrared (SWIR) photodetector that serves as a component in a SWIR imaging system.

2. The photodetector of claim 1, wherein the first doping concentration is between 10.sup.15 cm.sup.−3 and 10.sup.20 cm.sup.−3.

3. The photodetector of claim 1, wherein the first doping concentration is about 10.sup.18 cm.sup.−3 and wherein the second doping concentration is about 10.sup.16 cm.sup.−3.

4. The photodetector of claim 1, wherein the first doping concentration is about 10.sup.18 cm.sup.−3, the second doping concentration is about 10.sup.16 cm.sup.−3, the first layer thickness is about 1 μm and the second layer thickness is about 10 μm.

5. The photodetector of claim 1, wherein the photodetector has a dark current that is smaller by about three orders of magnitude than a dark current of a Ge homojunction with similar doping concentrations under a reverse bias of between −0.1 V and 1 V.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Non-limiting examples of embodiments disclosed herein are described below with reference to figures attached hereto that are listed following this paragraph. Identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. The drawings and descriptions are meant to illuminate and clarify embodiments disclosed herein and should not be considered limiting in any way. In the drawings:

(2) FIG. 1 illustrates schematically in a side view a PN GaAs homojunction;

(3) FIG. 2 illustrates schematically in a side view a PN Ge homojunction;

(4) FIG. 3 illustrates schematically in a side view the disclosed PN Ge—GaAs heterojunction;

(5) FIG. 4 shows the calculation of reverse current of GaAs PN homojunction, Ge PN homojunction and the disclosed Ge—GaAs PN heterojunction;

(6) FIG. 5 shows the calculation of the electric field at the Ge and GaAs interface of the disclosed Ge—GaAs PN heterojunction;

(7) FIG. 6 shows the calculated conduction and valence edges along with the Fermi level at the disclosed PN Ge—GaAs heterojunction at equilibrium;

(8) FIG. 7 shows the calculated reverse dark and light current of the disclosed Ge—GaAs PN heterojunction while being illuminated by 1.31 um light signal at 100 mW/cm.sup.2.

DETAILED DESCRIPTION

(9) Disclosed herein are PN Ge/GaAs heterostructure-based photodiodes. It is determined that in such PDs, when the doping of Germanium is significantly higher than the doping of the GaAs, the space charge region (depletetion layer) is located within the GaAs, and thus the dark current tends to be equivalent to that of a GaAs homojunction, where the SRH current is minimized due to a low intrinsic carrier concentration. As used herein, “significantly higher doping” refers to a doping higher by at least one order of magnitude.

(10) The heterojunction energy band is engineered such that there is no conduction band edge discontinuity for minority carriers (electrons), providing good photo-responsivity.

(11) Another advantage of this structure is for passivation. Indeed as the space charge is in the GaAs, passivation technologies may be applied to GaAs instead of to Ge.

(12) By considering P type Ge and N type GaAs, with doping of the Ge significantly higher than that of the GaAs, we show that the dark current will be reduced dramatically compared to that in a homojunction, and in parallel the band discontinuity at the conduction band can be made sufficiently small, in the range of few KT, such that efficient transport of electrons to reach the space charge region can be made possible, giving rise to high photoconductivity. A graded doping can be further used to assist the carriers transport by implementing a drift mechanism in the Germanium layer.

(13) Numerical simulations were implemented using a one-dimensional model on a computer program calculating carrier transport in semiconductors, implementing a drift equation, a diffusion equation, continuity equations, Poisson equations and boundary conditions. Simulation below are performed with the Ge—GaAs system.

(14) FIGS. 1, 2 and 3 illustrate schematically in side views respectively a known PN GaAs homojunction, a known PN Ge homojunction and the PN Ge—GaAs heterojunction disclosed herein. The P and N layers are marked respectively by 102 and 104 in FIGS. 1, 202 and 204 in FIGS. 2 and 302 and 304 in FIG. 3.

(15) For all simulations below, the P type doping concentration was set to 10.sup.18 cm.sup.−3 and the N type doping concentration was set to 10.sup.16 cm.sup.−3. The P layer thickness was set to 1 μm and the N type layer thickness to 10 μm. Note that these values serve as an example, and actual values may deviate from these values, depending on material properties, geometries, and optimization of the desired functionality. For example, the Ge layer doping may vary between 10.sup.15 cm.sup.−3 and 10.sup.20 cm.sup.−3, while the doping of the GaAs layer may vary between 10.sup.13 cm.sup.−3 and 10.sup.19 cm.sup.−3. For example, the Ge layer thickness may be between a few hundreds of nanometers (nm) and up to hundreds of micrometers (μm), preferably a few μm. The GaAs layer thickness may be from 1 μm to a few hundreds of μm, preferably a few hundred μm.

(16) FIG. 4 shows calculated reverse dark currents of the Ge PN homojunction (line 402) the GaAs PN homojunction (line 404), and the disclosed Ge—GaAs PN heterojunction (line 406). One can see that the dark current of the disclosed Ge—GaAs heterojunction is drastically reduced compared to the dark current of the Ge homojunction, and it is very close to the dark current of a GaAs PN homojunction.

(17) The profile of the electric field 502 created at the Ge—GaAs interface between layer 302 and layer 304 is depicted in FIG. 5. One can clearly see that electric field 502 is almost entirely located within GaAs layer 304, indicating that the junction space charge (depletion region) is located in GaAs layer 304. This explains the dramatic reduction in dark current as observed in FIG. 4, by the reduction of the diffusion and SRH mechanisms.

(18) FIG. 6 shows the calculated conduction band 602 and valence band 604 vs. the junction depth for the disclosed PN Ge—GaAs heterojunction at zero bias. A conduction band edge discontinuity 606 is indicated at the interface between Ge layer 302 and GaAs layer 304. One sees that the conduction band discontinuity is negligible and does not constitute a significant blocking structure for blocking electrons from passing from the Ge to the GaAs under zero and reverse biases, thus allowing a good photoresponse of the disclosed device. In an example, the discontinuity is 70 meV at 0V and is 60 meV at 1V bias.

(19) FIG. 7 shows calculated dark and light currents for the disclosed PN Ge—GaAs heterojunction. Curve 702 represents the photocurrent of the disclosed Ge—GaAs heterojunction with a wavelength of 1.31 μm at an exemplary optical power density of 100 mW/cm.sup.2. One can easily see that the photo-responsivity is good even at zero bias. The photo-responsivity can be further improved by optimizing the parameters of the photodiode (e.g. doping, dimensions, etc.). Same is true for the dark current, which can be further reduced by such optimization.

(20) Unless otherwise stated, the use of the expression “and/or” between the last two members of a list of options for selection indicates that a selection of one or more of the listed options is appropriate and may be made.

(21) It should be understood that where the claims or specification refer to “a” or “an” element, such reference is not to be construed as there being only one of that element.

(22) While this disclosure has been described in terms of certain embodiments and generally associated methods, alterations and permutations of the embodiments and methods will be apparent to those skilled in the art. The disclosure is to be understood as not limited by the specific embodiments described herein, but only by the scope of the appended claims.