R. D. Burnham

Threshold Requirements and Carrier Interaction Effects in GaAs Platelet Lasers (77°K)

The experimentally observed thresholds for laser operation of optically pumped GaAs platelets are presented. The measurements were performed on uniformly doped samples of p‐type, n‐type, and compensated crystal (doping range 1014/cm3≤n, p≤1020/cm3). The observed thresholds are considerably lower than previously measured or predicted and exhibit no essential dependence upon impurity concentration. The 16‐meV shift of laser photon energy from bandgap in lightly doped GaAs is ascribed, as before, to electron‐hole‐lattice (EHL) interactions. Based upon the results of platelet laser threshold data and photon energy data, a comparison is made of the relative strength of the various EHL interaction mechanisms. If Coulomb interaction and dielectric screening can be ignored, over a certain doping range a remarkable fit of the photon energy data is afforded by the electron‐electron interaction mechanism.

Spontaneous and Stimulated Carrier Lifetime (77°K) in a High‐Purity, Surface‐Free GaAs Epitaxial Layer

A simple optical transmission and excitation technique is demonstrated for the measurement of extremely short excess carrier lifetimes in GaAs. The middle layer of an epitaxial n+/n/n+ sample, in which excess carriers tend to be confined to the center of the structure, is selectively excited optically and serves essentially as a surface‐free sample that exhibits the recombination properties intrinsic to bulk material. Excited from lower to higher level (77°K), this layer (Nd∼2−3×1015/cm3, μ=6340 cm2/V sec) varies continuously in carrier lifetime from τ≈10−9 sec (spontaneous recombination) to τ≈2×10−10 sec (stimulated recombination), as determined by the measured change in sample absorption caused by Burstein shift of the carrier quasi‐Fermi levels (carrier population). The carrier distribution and generation rate are shown to be quite uniform within the sample as a result of the inverse dependence of the absorption upon the carrier concentration.

AlxGa1−xAs1−y′P y′–GaAs1−yPy HETEROSTRUCTURE LASER AND LAMP JUNCTIONS

A method is described to grow successfully from solution AlxGa1−xAs1−y′P y′ (p‐type) on GaAs1−yPy (n‐type), to preserve the lattice match (y′ ≈ y), and to obtain the improved heterostructure junction devices previously realized only in the AlGaAs/GaAs system. Although not optimized, these structures have been operated as pulsed room‐temperature lasers, and because of an inherent wide‐gap window can be used conveniently for optical purposes and for excess carrier lifetime measurements. Carrier lifetime measurements, for example, indicate freedom from defects at the AlGaAsP/GaAsP barrier.

SPECTRAL BEHAVIOR, CARRIER LIFETIME, AND PULSED AND cw LASER OPERATION (77 °K) OF In1−xGaxP

The spontaneous and laser spectral behavior of high‐quality In1−x Gax P grown slowly from an In solution in a small temperature gradient is described. The band‐to‐band carrier decay times are measured below (2.3 nsec, 77°K) and above (0.85 nsec) laser threshold by an optical phase shift technique and indicate low bulk and surface losses (s≤104 cm/sec). Continuous laser operation of a thin In1−x Gax P sample is demonstrated at a photoexcitation level of ∼103 W/cm2 (hνpump =1.96 eV).

Double Heterojunction AlGaAsP Quaternary Lasers

Stimulated emission in AlGaAsP is demonstrated. Double heterojunction AlGaAsP lasers grown from Ga solution on GaAsP substrates exhibit 300 °K thresholds as low as 104 A/cm2 (∼ 8450 A) and shift ∼ 450 A to shorter wavelength at 77 °K. In spite of its more complicated crystalline structure, solution‐grown AlGaAsP appears to exceed in quality vapor‐grown GaAsP.

STIMULATED EMISSION IN In1 ‐xGaxP

By a modified Bridgman solution‐growth technique employing a small temperature gradient. InP and GaP source crystal are used to saturate an In solution at ∼ 925°C and grow In1−xGaxP (x ∼ 0.3) at ∼ 925°C. This material is shown to exhibit stimulated emission.

Stimulated Emission Involving the Nitrogen Isoelectronic Trap in GaAs1−xPx

Earlier work on III–V semiconductor lasers has shown that stimulated emission occurs on band‐to‐band transitions or via transitions involving donors or acceptors. Based on the behavior of the N isoelectronic trap in promoting efficient carrier recombination in GaP, the present work shows that the N isoelectronic trap in GaAs1−xPx is distinct and over part of the composition range x it can be operated in recombination in the stimulated emission (laser) regime, thus establishing a fundamental new laser transition in a III−V semiconductor. These results (4.2 and 77°K) are obtained by means of optical pumping of lightly doped n‐type samples of estimated high nitrogen concentration.

DIRECT OBSERVATION OF A DYNAMIC BURSTEIN SHIFT IN A GaAs:Ge PLATELET LASER

Data (77°K) are presented showing a dynamic Burstein shift of the absorption edge during spontaneous and laser operation of GaAs:Ge platelets. This is indicated directly by an increase in the relative transmission of the pump power through the sample with increased pump excitation. By the use of pumps with photon energies (1.527 eV) closely matched to the GaAs absorption edge (1. 513 eV), changes in the relative transmission of from 4 to 30% have been achieved. The data presented also show a change in the rate of decrease of the absorption with the onset of stimulated emission by the platelet.

Optical phase shift measurement (77°K) of carrier decay time in direct GaAsP☆

Abstract Measurements are made (77°K) of excess carrier lifetimes and the spectral behaviour of optically excited high quality vapor grown n-type GaAs1-xPx (0.0667 ⩽ x ⩽ 0.35) for both the low level spontaneous and the high level laser regimes of excitation. In several platelet samples the ambiguities in bulk lifetime measurements resulting from surface recombination are minimized by two methods, one involving excitation through a GaAlAsP window and the other a phosphorus anneal of the sample. The spontaneous lifetime of n-type GaAsP with reduced non-radiative surface losses is typically 1–1.6 nsec which may be compared to spontaneous lifetimes of 4–5nsec (Δn ∼ 1016/cm3) for lightly doped n-type GaAs. This indicates that additional bulk non-radiative processes occur in the ternary probably arising from the disorder and strain in the alloy system.

GaAs junction lasers containing the amphoteric dopants Ge and Si

Abstract Laser operation ofp-n junctions in GaAs doped with amphoteric impurities is demonstrated. The previous limitation to laser operation of such junctions caused by the broad spatial spread of the injected carriers (electrons), and their consequent low density at a given current level, can be overcome with a heterojunction reflecting barrier juxtaposed within a diffusion length of the junction. The effect of the barrier is great enough to allow significant stimulated emission to occur in GaAs (p-type GaAs : Ge and GaAs : Si) not previously lased in junction structures, and at current levels 10 × lower than those that have been previously inadequate.