This invention relates to conversion of quantum well structures from one type to another type with higher optical transition strength.
A semiconductor quantum well (QW) can be broadly classified as Type 1 (electron and hole wave functions overlap substantially) and Type 2 (electron and hole wave functions are substantially non-overlapping). Either type of QW relies upon decay of (free) electrons into (vacant) holes and thus upon overlap of the electron and hole wavefunctions near the recombination region so that a Type 2 QW structure will generally have a much smaller optical gain or associated electron-hole current than does a Type I QW structure. A semiconductor material such as InAs has an attractive band gap in the mid-infrared region, but materials such as AlSb/InAs in a conventional configuration appear to have only Type II band edge lineups. This precludes use of InAs, in a conventional QW configuration, from being used to produce, or to detect, light of mid-infrared wavelengths.
What is needed is an approach for converting a Type II QW material in to Type I QW material. Preferably, this approach should be flexible and should provide a spectrum of emission wavelengths in a selected wavelength band, such as a mid-infrared band. Preferably, this approach should work with a variety of choices of column III and column V semiconductor materials and with a continuum of geometric parameters associated with the QW wells. Preferably, this approach should allow use of the converted material for detection of the presence of light and/or for production of light in a specified wavelength band.
These needs are met by the invention, which provides an N-layer QW structure, with N≧5. In one embodiment, a five-layer semiconductor structure is provided, where the individual layers are doped, undoped, active, undoped and doped in that order, and the doping levels, location and widths of the two doped layers are chosen so that overlap of electron and hole wavefunctions are sufficiently strong to have high optical transition strength.
One example, using InAs for the active region and AlSb for the doped regions is found to provide a type 1 QW structure with adequate optical gain. The valence band offset between AlSb and InAs is taken to be 180 meV, which is the upper limit in the range of uncertainty. Proper doping in the heterostructure converts the InAs layer to a well for holes so that a Type II QW structure is thereby converted to a Type I QW structure. Both dipole moments and optical gain increase significantly with increasing doping.
In one embodiment, a five-layer QW semiconductor structure 10 is constructed as illustrated in
h(i)=z(i+1)−z(i)=zi,I+1(i=0,1, . . . ,5). (1)
The layer thicknesses are optionally the same.
The electrical charges associated with the five layers of the structure 10 are
h(1)=z(2)−z(1)=z1,2. (3)
h(3)=z(4)−z(3)=z3,4, (4)
where N1, N2 and N3 are the donor doping densities in layers 11-1, 11-3 and 11-5, with N2 optionally 0. Charge neutrality in the structure 10 requires that
z1,2N1+z5,6N3=z3,4N2. (5)
From Poisson's equation, the associated electrical fields E(z) within each of the layers 11-1, 11-2 and 11-3 are determined by
where ∈r (>1) is a relative dielectric parameter for the layer material (e.g., ∈r=12 and ∈r=15.15 for AlSb and InAs, respectively).
The electrical potentials V(z), illustrated in
The maximum barrier height for holes in the region z(3)<z<z(4) is estimated as Veff≈180 meV. At the interface z=z(3)−0,
V(z(3)−0)=e2Ndz1,2z2,3/(∈0∈b), (8)
and the hole barrier height at this interface will become 0 if V(z(3)−0)=Voff. The maximum value of the hole potential occurs at z=z(m)=(z(3)+z(4))/2, where
V(z(m);hole)=(e2z3,4/2∈0∈b){−(N1z1,2+N3z5,6)/4+N1z1,2}. (9)
The remaining portion of the QW layers, z(m)≦z≦z(6), is constructed by analogy to the first portion, z(1)≦z≦z(m). When V(z(m);total)=Veff, or V(z(m);total)>Veff, the total electrical potential V(z) behaves as illustrated in
By reducing or eliminating the hole barrier at the interface z=z(3) (and similarly at z=z(4), the hole wavefunctions are permitted to substantially overlap with the electron wavefunctions, and the structure 10 in
The QW structure shown in
The invention described herein was made in the performance of work under a N.A.S.A. contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, as amended, Public Law 85-568 (72 Stat. 435; 42 U.S.C. §2457).
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