Differential carrier lifetime in oxide-confined vertical cavity lasers obtained from electrical impedance measurements

Abstract

Differential carrier lifetime measurements were performed on index-guided oxide-confined vertical cavity surface emitting lasers operating at 980 nm. Lifetimes were extracted from laser impedance measurements at subthreshold currents, with device size as a parameter, using a simple small-signal model. The carrier lifetimes ranged from 21 ns at 9 A, to about 1 ns at a bias close to threshold. For a 6ϫ6 m 2 oxide aperture device the threshold carrier density was n th ϳ2ϫ10 18 cm Ϫ3 . The effect of carrier diffusion was also considered. An ambipolar diffusion coefficient of D ϳ11 cm 2 s Ϫ1 was obtained. © 1999 American Institute of Physics. ͓S0003-6951͑99͒00507-0͔Carrier lifetime measurements are an important tool in semiconductor laser characterization. 1,2 Recently, a new electrical technique for differential carrier lifetime measurements was developed. 2 It originally applied to edge emitting lasers with bulk active region. More recently, it has been applied to vertical cavity surface emitting lasers ͑VCSELs͒ 3 with quantum wells ͑QWs͒ in the active region. In this letter, experimental results on differential carrier lifetimes in indexguided oxide-confined VCSELs are reported. Lifetimes were obtained from laser impedance measurements at subthreshold currents and the equivalent circuit modeling. Threshold carrier densities and the corresponding recombination parameters were calculated as a function of device size. In addition, the effect of lateral carrier diffusion out of the active region was also considered.Devices studied in this work were oxide-confined InGaAs VCSELs lasing at 980 nm. The current aperture was obtained by partial oxidation of two quarter-wavelength GaAlAs layers. 4 Lasers with aperture sizes in the range of 6-12 m had room temperature threshold currents from 220 to 600 A. The laser impedance was measured from 1 MHz up to 3 GHz. Measurements were performed for bias currents between 9 A and threshold. Figure 1 presents the frequency dependence of the measured impedance of a device with a ϳ8ϫ8 m 2 active region, with a threshold current I th Ϸ240 A, at 40 A of drive current. The measured laser impedance was modeled using the small-signal subthreshold equivalent circuit shown in the inset of Fig. 1. The parameters included in this model were the following: an inductance L associated with a bond wire, a parallel combination of a resistance R m and capacitance C m associated with the top mirror interfaces, and a parallel combination of a resistance R d and a capacitance C a representing the active region. The impedance of the bottom mirror was neglected. The total impedance of the circuit can be written aswhere d ϭR d C a is the characteristic time equal to the differential carrier lifetime. 5,6 This model does not consider the carrier transport and capture dynamics discussed elsewhere 7,8 because it was assumed that the capture time is small and the escape time from the wells is very large compared to the carrier lifetime in the wells. In this simple model, d is the effective lifetime of the carri...

Honeycomb photonic crystal vertical-cavity surface-emitting lasers: coupled cavities enhancing the single-mode range

“…An excess absorption loss of 10 cm −1 has been assigned to the DBRs for all optical modes. A radiative recombination coefficient of 1.3 × 10 −10 cm 3 ∕s and an Auger coefficient of 4 × 10 −29 cm 6 ∕s have been selected [35]. Carriers have been allowed to spread laterally in the QWs, having a diffusion coefficient of 10 cm 2 ∕s.…”

Network Analyzer for Carrier Lifetime Measurements in Mid-IR Semiconductor Lasers

“…1) by using the technique reported in [2,3]. The total carrier density, n, and carrier lifetime % were obtained from the measured t d using «(/) = -J 0 r d (I )dt and?…”

Small-signal impedance characteristics of quantum-well laser structures

“…In particular, the bandwidth of quantum-well ͑QW͒ lasers can be limited by long carrier transport and capture times or by high escape rates from the wells. 8 In our small-signal model, the intrinsic laser impedance was represented by a parallel RC circuit with a characteristic time constant equal to the differential carrier lifetime d . Kan and Lau 1 were first to show that a set of rate equations can be used to construct an equivalent circuit of the QW laser and to extract characteristic time constants from frequency dependent electrical impedance measurements.…”

“…8 This type of expression also applies to bulk lasers 7 where the bulk active region substitutes the well. 8 This type of expression also applies to bulk lasers 7 where the bulk active region substitutes the well.…”

“…In Fig. Another important conclusion is that the technique of estimating differential lifetime and threshold carrier density, based on impedance measurements, 7,8 is applicable to the QW as well as bulk lasers. The values of w and cap for different drive currents were extracted by fitting the impedance with Eq.…”