James E. Ehret

Photoluminescence studies of beryllium doped GaAs/AlGaAs quantum wells

Abstract A photoluminescence (PL) investigation of beryllium doped GaAs/AlGaAs multiple quantum wells is reported. MBE grown samples with well widths 30–75 A and barrier thicknesses 100–500 A are included. The effect of beryllium doping in the well region of sheet carrier density 3×10 11 to 4×10 12 cm −2 on the position of the first conduction band-to-first heavy hole band (C1–HH1) free exciton line is investigated. The position of PL peak energies as a function of well width and doping is calculated using single particle energies from an envelope function approximation calculation and an estimate of many body effects, including a two-dimensional, screened exchange interaction. A very good agreement is found between the calculated and measured PL peak energies.

Gigahertz modulation of GaAs-based bipolar cascade vertical cavity surface-emitting lasers

The high-frequency modulation characteristics of GaAs-based bipolar cascade vertical cavity surface-emitting lasers operating at 980 nm with GaAs tunnel junctions and p-doped Al0.98Ga0.02As oxide apertures have been measured. We achieve -3 dB laser output modulations of 6.5 GHz for two-stage and 9.4 GHz for three-stage devices in response to small-signal current injection at an operating temperature of -50 degrees C.

Overcoming absorption saturation with doping in p-type quantum well infrared photodetectors: modeling and experiment

Bound-to-continuum normal-incidence absorption in p-type GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) is strongest when the second light-hole (LH2) level is resonant with the top of the valence band QW. However, we found that such absorption saturates as a function of doping in the well. Using the envelope-function model (EFA), this paper shows that moving the LH2 resonance slightly deeper into the continuum avoids absorption saturation and produces optimal p-QWIP response. A suitable set of mid-IR samples was grown to test this conjecture and their photoresponse measured. The results indicate that absorption can be more than doubled through the use of the new p-QWIP designs. This result is explained by showing that the line of resonances in the continuum as a function of the in-plane wave vector eventually becomes a bound LH2 band in the well at some critical wave vector. Therefore, it is possible to avoid absorption saturation by matching this critical wave vector (i.e., well width and/or well depth) with the Fermi wave vector (i.e., doping in the well) for the desired QWIP (i.e., cutoff wavelength).

Gain and luminescence modeling for high-power laser applications

A general scheme for the determination of vital operating characteristics of semiconductor lasers from low intensity photo-luminescence spectra is outlined and demonstrated. A fully microscopic model for the optical properties is coupled to a drift-diffusion model for the mesoscopic charge and field distributions to calculate luminescence and gain spectra in barrier-doped laser material. Analyzing experiments on an optically pumped multi quantum-well structure it is shown that the electric fields arising from the charges of ionized dopants lead to strongly excitation dependent optical properties like significant differences between luminescence and gain wavelengths.

Gain and photoluminescence in semiconductor lasers

A general scheme for the determination of vital operating characteristics of semiconductor lasers from low intensity photo-luminescence spectra is outlined and demonstrated. A fully microscopic model for the calculation of optical properties is coupled to a drift diffusion model for the mesoscopic charge and electric field distributions to calculate photo-luminescence and gain spectra in barrier-doped semiconductor laser material. Analyzing experiments on an optically pumped multi quantum-well structure it is demonstrated that the electric fields arising from the space charges of ionized dopants contribute to strongly excitation dependent optical properties, such as significant shifts of the luminescence versus peak gain wavelengths.

Quantum well infrared detection devices

Quantum wells, especially those made of GaAs and InP related compounds, have enabled several unique infrared devices. Two prime examples are quantum well infrared photodetectors (QWIP) and quantum cascade lasers. This paper discusses a few examples of QWIP related devices: (1) QWIPs are well suited for high speed and high frequency applications--work on achieving high absorption efficiency and high operating temperature has been carried out. (2) A variation of conventional QWIP structures can lead to simultaneous visible and infrared detection, and demonstrations using both GaAs and InP based structures have been made. (3) P- type structures may achieve competitive performance and lend to easy fabricating of large focal plane arrays, and good performance has been achieved in resonant-cavity enhanced p- QWIPs.

Electrically-Tunable Group Delays Using Quantum Wells in a Distributed Bragg Reflector

We present an optical delay line structure incorporating InxGa1-xAs quantum wells in the GaAs quarter- wave layers of a GaAs/AlAs distributed Bragg reflector. Applying an electric field across the quantum wells shifts and broadens the e1-hh1 exciton peak via the quantum- confined Stark effect. Resultant changes in the index of refraction thereby provide a means for altering the group delay of an incident laser pulse. Theoretical results predict tunable delays on the order of 50 fs for a 30-period structure incorporating 3 quantum wells per GaAs layer. Structure design, growth and fabrication are detailed. Preliminary group delay measurements on large-area samples with no applied bias are presented.