P. Blood

A correlation between electron traps and growth processes in n‐GaAs prepared by molecular beam epitaxy

By using a single Knudsen source independently to generate molecular beams of As2 and As4 from elemental arsenic, a direct correlation has been observed between the arsenic species used and deep level concentrations in molecular beam epitaxy grown n‐type GaAs films. Some preliminary observations on comparative growth temperature effects and Ge site occupancy are also reported.

Measurement and calculation of spontaneous recombination current and optical gain in GaAs-AlGaAs quantum-well structures

Experimental determinations have been made of the peak optical gain as a function of spontaneous recombination current density for GaAs quantum wells of width 25 and 58 A bounded by AlGaAs barriers. These data were obtained from measurements of spontaneous emission spectra, observed through narrow windows in the 50‐μm‐wide contact stripes of oxide isolated lasers, using only a single reference value of the optical absorption coefficient above the band edge to calibrate the measurements in absolute units. These results are in good agreement with gain‐current curves calculated using a model which includes unintentional monolayer well width fluctuations, band‐gap narrowing and intraband carrier‐carrier scattering. The characteristic intraband scattering time is calculated from first principles as a function of electron energy and carrier density on the basis of a 2‐dimensional Auger‐type process. This lifetime gives a much better representation of our observed spontaneous spectra than a lifetime which is sim...

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. >

Influence of broadening and high-injection effects on GaAs-AlGaAs quantum well lasers

Gain spectra and gain-current relations are calculated for GaAs-AlGaAs quantum-well lasers using a model which incorporates a phenomenological description of bandgap narrowing due to many-body effects at high injection, transition broadening by a carrier-density-dependent intraband scattering process, and broadening of the density of states function by fluctuations in the well width. The justification for including all these phenomena is made by examining spontaneous emission spectra observed through contact windows on quantum-well lasers. >

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.

Temperature dependence of threshold current in GaAs/AlGaAs quantum well lasers

We have calculated the threshold current and its temperature (T) dependence in the range 200–400 K for AlGaAs quantum well lasers with 25‐A‐wide GaAs wells using a model which includes lifetime broadening of the transitions and broadening of the density of states function by fluctuations in the well width. The threshold current varies approximately linearly with T and the principal effect of broadening is to increase the threshold current causing a reduction in the fractional change of current with temperature. The apparent value of the parameter T0 is increased to ≊400 K, compared with ≊320 K without broadening. The calculations are compared with experimental data.

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.

Temperature dependence of spontaneous emission in GaAs-AlGaAs quantum well lasers

Using quantum well laser devices with a window in the p‐type contact, we have measured the relative change of spontaneous emission intensity at threshold with temperature for 58‐A‐wide GaAs wells. Over the range 250–340 K the data are in good agreement with the linear relation obtained from a gain‐current calculation which includes transition broadening. This linear behavior contrasts with the stronger temperature dependence of the total measured threshold current of the same devices which includes nonradiative barrier recombination processes.Using quantum well laser devices with a window in the p‐type contact, we have measured the relative change of spontaneous emission intensity at threshold with temperature for 58‐A‐wide GaAs wells. Over the range 250–340 K the data are in good agreement with the linear relation obtained from a gain‐current calculation which includes transition broadening. This linear behavior contrasts with the stronger temperature dependence of the total measured threshold current of the same devices which includes nonradiative barrier recombination processes.

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.