E. D. Fletcher

Influence of the barriers on the temperature dependence of threshold current in GaAs/AlGaAs quantum well lasers

Using window devices, light emission has been observed from the barrier regions of lasers with 25-A-wide quantum wells. From measurements of threshold current as a function of temperature on devices grown by molecular-beam epitaxy using different Al cells for the barriers, the strong influence of nonradiative barrier recombination processes on the threshold current has been demonstrated. Further measurements of threshold current as a function of hydrostatic pressure show that recombination from the L and X conduction-band minima makes an important contribution to the current. The calculations show how the temperature dependence of threshold depend on factors such as cavity length and the number of quantum wells. >

Short wavelength (visible) GaAs quantum well lasers grown by molecular beam epitaxy

GaAs‐AlGaAs multiple quantum well (MQW) injection lasers with well widths from 55 to 13 A have been grown by molecular beam epitaxy and operated at room temperature, showing emission at wavelengths down to 704 nm, the shortest reported for a MQW injection laser with GaAs wells. In a device with 25‐A wells some evidence of coupling was apparent when barrier widths were reduced to 40 A. For devices with 80‐A barriers there is a difference of about 20 nm between the calculated n=1 (e–hh) transition wavelength and the lasing wavelength, whereas the calculation agrees with photovoltage absorption measurements on the same structures.

Emission wavelength of AlGaAs‐GaAs multiple quantum well lasers

We have recorded spontaneous emission spectra from multiple quantum well lasers grown by molecular beam epitaxy with 25‐A‐wide GaAs wells by opening a window in the top contact stripe. These spectra have a low‐energy tail and consequently the gain spectra derived from them show that laser emission occurs at a lower photon energy than the lowest energy confined particle transition. The observed laser wavelength and threshold current are consistent with the position of the peak in the gain spectrum.

Dependence of Threshold Current on the Number of Wells in AlGaAs-GaAs Quantum Well Lasers

GaAs‐AlGaAs multiple quantum well injection lasers have been grown by molecular beam epitaxy with different numbers (N) of uncoupled GaAs wells 25 A wide symmetrically disposed about the center of a 4000‐A‐wide waveguide. The devices emit at about 770 nm and for N=4 the broad area threshold current density is 1.1 kA cm−2. The threshold current increases with increasing N (2<N<40) and this can be accounted for by changes in the optical confinement factor and the active ‘‘volume,’’ which implies that changes in capture probability with N in this structure are small.

Substrate temperature dependence of SQW alloy and superlattice lasers grown by MBE using As2

Abstract Until recently most semiconductor device structures grown by MBE have used As 4 . In such structures during the growth of (Al,Ga)As alloys a characteristic roughness is observed under some growth conditions. The threshold current of lasers grown using As 4 is also strongly influenced by the choice of substrate temperature. It is possible therefore that these two factors are related. We have compared the morphology of thick films of Al 0.5 Ga 0.5 As grown using As 2 and As 4 over the temperature range 610 to 710°C. For films grown with As 4 the characteristic roughness in such structures is observed but over the whole temperature range films grown with As 2 have a specular appearance. We have grown, using As 2 , a series of graded refractive index separate confinement heterostructure single quantum well (57 A) lasers with and without pre-layers at temperatures from 610 to 710°C. In this series of samples the Al and Ga fluxes were kept constant, the nominal Al fractions for the outer confinement and barrier regions were 50% and 25%, respectively. A similar set of stepped separate confinement structures with prelayers was grown for comparison. In a fourth series of structures the separate confinement and part of the outer cladding regions were replaced by short period superlattices. Contrary to previous reports for structures grown using As 4 , no strong dependence of threshold current on growth temperature was observed. Results for four sets of lasers will be discussed in detail. An additional benefit of using As 2 is the reduced loss of Ga at high temperatures due to a reduction of free metallic group III element present on the surface during growth.

Spontaneous recombination current in InGaAs/GaAs quantum well lasers

We have studied the intrinsic factors which determine the threshold current and its temperature dependence in 160‐A‐wide In0.2Ga0.8As single well quantum lasers with GaAs barriers, grown by molecular beam epitaxy on GaAs substrates. By measuring the relative temperature dependence of the spontaneous emission intensity at threshold we show that radiative transitions between higher order (n=2,3) electron and heavy hole subbands make a significant contribution to the threshold current and its temperature sensitivity, even in devices where the laser transitions are between n=1 subbands. These higher transitions will also influence the dependence of threshold current and its temperature sensitivity on well width.

Observations of barrier recombination in GaAs‐AlGaAs quantum well structures

Using laser structures with a window in the contact stripe, we have observed recombination from the wells and barrier regions of GaAs‐AlGaAs quantum well lasers. The magnitude of the ratio of emission intensities from the barrier and the well, and the dependence of this ratio upon injection current, are in good agreement with a calculation in which the carrier populations in well and barrier are in thermal equilibrium at the lattice temperature (300 K).

Short‐period (AlAs)(GaAs) superlattice lasers grown by molecular beam epitaxy

We have used short‐period all‐binary (AlAs)(GaAs) superlattices with layers as thin as three monolayers to synthesize the barrier and cladding regions of GaAs quantum well lasers grown by molecular beam epitaxy. By studying the threshold current of single‐ and double‐well devices as a function of cavity length and temperature, we conclude that the optical scattering losses are very low, that the gain‐current characteristics are similar to alloy barrier devices, and that there is evidence for current leakage by recombination in the barriers.

Pressure dependence of the nonradiative lifetime in GaAs/AlGaAs double‐heterostructure lasers

We have measured the linear losses in a GaAs/AlGaAs double‐heterostructure laser as a function of applied hydrostatic pressure. The nonradiative lifetime increases by a factor 2.5 from zero to 7.6 kbar, and if this is due to recombination via a deep state, then the rate‐limiting capture cross section decreases with pressure, suggesting capture by multiphonon emission. We find that it is necessary to postulate the existence of other nonlinear losses at threshold to account for the increase in threshold current with pressure measured on the same structure. Carrier transfer to higher conduction‐band minima in the active or cladding regions are suggested as possible mechanisms.

Short wavelength (visible) quantum well lasers grown by molecular beam epitaxy

Abstract We have fabricated AlGaAs multiple quantum well lasers from a variety of structures grown by molecular beam epitaxy with the objective of achieving operation at a short wavelength with GaAs wells. A series of structures with well widths from 55A down to 13A gave pulsed room temperature laser operation at wavelengths from 837nm to 707nm. All the devices operated at longer wavelengths than that calculated for the n=1(e-hh) transition, though from measurements of their electroluminescence spectra at currents as low as 7% of threshold we find no evidence for changes in the sub-band separation at high injection. The threshold current density of a simple broad area MQW device with 160A wide GaAs wells operating at ≈880nm was 1.2kA cm -2 and an analysis of the threshold current density and losses in these device suggests that interface optical scattering is small.