Ley de Recursos Hídricos – Ley No.29338

Having studied the spectral properties of VCSELs we now go on to look at the output power characteristics. These are, in general, the most important properties of a device and strongly dictate the applications for which the laser may be viable. We begin by looking at the power output versus current injection (L-I) characteristics of VCSELs of various dimensions fabricated from sample C. The reader will recollect that this sample was designed predominantly for a low threshold current.

.30 20pm device .20 .15 .05 .00 10 15 20 25 0 5 50um device ^ .0 O 0.5 Current (mA) 8 k 0> o a s a 3 o 10 9 100pm device 8 7 6 5 4 3 2 1 0 200 300 0 100 20 40 60 80 Current (mA)

Measurements are taken using 1 ps pulses, with a duty cycle of 1:1 0 0. Thermal turnover at high injection is still evident. Loss of light due to the reflection from the glass slide (mount) and detector surface is not accounted for.

Current (mA)

Figure 4.22 Pulsed light output vj. current input curves f o r sample C.

The output power curves for 20, 50 and 100pm VCSELs processed from wafer C are shown in figure 4.22. It is noted that larger devices (200pm) did not lase due to the dominance of in-plane luminescence. All of the L-I curves in this figure show a distinct threshold characteristic and a relatively linear L-I curve above threshold. It is noted that none of the

emission spectra for these devices exhibit sidemodes, they are similar to those in figure 4.20. Most of the deviation, at high injection, from a linear L-I curve is due to inadequate heat- sinking. The samples are mounted onto glass which has a poor thermal conductivity. Unfortunately the pulse lengths, as discussed earlier, are limited to a minimum of Ips and this results in a degree of sample heating. Increasing the time between pulses (~100|is) lowers the power reaching the Si photodetector, thereby increasing the noise in the measurement, and has little effect on the turn-over characteristics. Predominantly the turnover in the L-I curves is due to a thermally induced shift in the gain spectmm of the QWs. This leads to a detuning of the peak gain and the FP resonance of the laser. Also, the increased temperature leads to an increase in the carrier leakage problems described earlier. Collectively these effects have been studied in detail, both theoretically and experimentally, by Scott and Geels and the reader is referred to the literature [Scott et al. ‘93a, Geels et al. ‘93] for further information.

Some of the important parameters to be derived from figure 4.22 are given in table H. The threshold currents of all devices are seen to be below 100mA. The corresponding threshold current densities are seen to be relatively constant and of order IkA/cm^. A slight increase in the threshold current density of the smallest device is attributed to the larger diffraction losses, sidewall scattering (of photons) and recombination (of electrons) that is known to occur within mesas of smaller diameter. The table also shows the threshold current density per QW. This value is used as a figure of merit, however it must be remembered that this quantity is merely an approximation to the injection level within each QW. Finally the output (conversion) efficiency is also shown. It is seen to increase with device area. This is again attributed to larger losses (for both electrons and photons) within the smaller devices.

device size Ith (mA) Jth (A/cm^) Jth / Nqw Pmax output efGciency

2 0pm 5.6mA 1783 594 A/cm^ 0.29mW 0.033mW/mA

50pm 20mA 1019 340 A/cm^ 4.1mW 0.075mW/mA

1 0 0pm 87mA 1108 369 A/cm^ 9.1mW 0.084mW/mA

Table II Table showing device characteristics fo r sample C.

Pmax is the power output attained at turnover.

Because the lasing wavelength in these samples is approximately 950nm the QWs within the cavity, which was designed for resonant gain at 980nm emission, will not coincide exactly with the antinodes of the standing wave optical field. This offset in wavelength is due to the QW peak gain falling at approximately 950nm, a growth error relating to the indium concentration in the QWs is the cause. The position, in wavelength, of the peak gain is determined by measuring

the thresholds of VCSELs processed from various areas of the wafer (thereby emitting at a range of wavelengths), this is discussed in more detail later.

It is noted that each of the VCSELs produced from sample C also work in a continuous wave (CW - un-pulsed) mode, at room temperature. Unfortunately the lack of heatsinking in this case limits the maximum output power to less than lOpW, even for the largest 100pm devices. Generally it is the thermal turnover and, in some cases, catastrophic failure that ultimately limits the CW power output.

The similarity in threshold current density between the various devices from wafer C leads us to expect that the power output of the devices may also scale simply with the area of the device. For the larger devices this certainly appears to be the case. Figure 4.23 shows the output power density (output power / device area) plotted against the injection current density.

2.5

^

Current density (mA/pm )

Figure 4.23 Pulsed light output vs. current input curves fo r sample C. All curves are normalised to their respective device area.

The 50pm and 100pm device curves clearly show a similar threshold and output characteristic up to injection densities of 0.02mA/pm^ beyond which the two curves diverge. The discrepancy with the 20pm device is caused by the increased surface scattering and recombination for that device. Divergence of the curves for the two larger devices may be attributed to the higher transverse modes, in-plane luminescence and the different heat dissipation properties of the 1 0 0pm laser.

Having looked at the power characteristics at a fixed emission wavelength, we may now use the wafer non-uniformity to study devices emitting over a range of wavelengths. It is noted that, due to the slow dependence of gain on QW well width, the gain spectra for all of the samples is essentially fixed. The varying parameter is thus the FP resonance wavelength which may be tuned across the QW gain spectrum by taking devices from different regions of the wafer

(see figures 4.4 and 4.18). Figure 4.24(a) shows the measured L-I curves for a number of 50|iim VCSELs emitting at a range of different wavelengths (taken from sample C). The figure clearly shows an increase in the threshold current of the devices as the wavelength is increased. A corresponding decrease in the devices maximum power output and conversion efficiency is observed. This effect may be completely attributed to a mismatch between the FP mode and the peak of the QW gain spectrum. This gain maxima, at the pulsed operating temperature of the devices, occurs at approximately 940nm. As the FP resonance passes this wavelength a sharp decrease in output power occurs, this is shown in greater detail in figure 4.24(b). In this way the shape of the QW gain spectrum may be determined [Sale et al. ‘92].

944nm increasing

SOpm device wavelength Ips pulses 1:100 duty cycle 968nm 0 10 20 30 40 50 60 Current (mA) 70 80 90 g 60 S 50 2 30 940 950 960 970 Wavelength (nm) Figure 4.24 Variation o f threshold with position on wafer.

The effect is due to a detuning o f the FP and gain peak.

The threshold current variation with emission wavelength, figure 4.24(b), shows a local minima between 940-950nm. Unfortunately, due to wafer uniformity, the lowest attainable wavelength for this sample is ~940nm. It is logical to assume that as the FP resonance passes through the gain spectrum maxima, to the short wavelength side, the threshold current will begin to increase, this has previously been observed by Sale and co-workers [Sale et al. ‘92]. This variation in the power characteristics of devices taken from a single wafer imposes limits upon the reproducibility and integration of VCSELs. For example large area arrays [Orenstein et al. ‘91] are often required with similar characteristics between devices, this will require highly uniform growth. Therefore the yield of a given growth technology will be dictated by its uniformity characteristics. On the plus side, the development of arrays emitting a range of wavelengths, for example for wavelength division multiplexing (WDM) applications, becomes a possibility [Hasnain et al. ‘91]. In this case however the wavelength separation between lasers

must again be small to prevent widely differing characteristics between devices. Overall these findings impose increased tolerances on the growth technology which must not only provide precise layer thicknesses and compositions but must also provide highly uniform layers. It is important to note that a number of dedicated industrial machines may already attain these high and exacting standards.

Having looked at the emission and power dependence of the resonant periodic gain sample (C) we may now study the power characteristics of sample D. This sample has a 3QW active region placed within a graded-index separate-confinement heterostructure, see appendix C (iv) for full details. This (GRINSCH) VCSEL is designed to prevent some of the carrier leakage problems evident in the power characteristics of sample C. The slightly reduced confinement factor in this case, due to the bunching of the QWs, should not degrade the performance (see figure 4.2). Furthermore the reduced output coupler reflectivity and the use of short-period superlattice grading within both n- and p-type DBRs should allow the attainment of higher output powers. 0.9 0.7 0.6 I 0.5 a 0.4 0.3 0.2 0.0 850 900 950 1000 1050 800 1100 Wavelength (nm)

Figure 4.25 Measured reflectivity spectrum fo r sample D (wafer centre). Roll-off on the stop-band is due to the detector and gold response.

The reflection spectrum for sample D, taken at wafer centre, is shown in figure 4.25. It demonstrates a FP resonance centred within the DBR stopband, the slight roll-off in the stopband reflectivity is an artefact of the measurement. This spectrum suggests that the use of a properly designed GRINSCH does not effect (detrimentally) the basic optical properties of the sample. In this case the FP resonance occurs at ~975nm, this is in excellent agreement with the design wavelength of 980nm (less than 1% discrepancy).

The L-I curves for devices of varying diameter are shown in figure 4.26. It is noted that the lasing characteristics of all devices processed from this sample are of the no-sidemode variety (see figure 4.20). This suggests a high degree of overlap of the spontaneous emission (and hence gain) spectrum with the DBR stop-band. In all cases the devices are taken from the centre of the wafer and are found to emit at wavelengths between 975-970nm, very close to the intended wavelength. Again this demonstrates the highly accurate growth.

18 16 14 12 10 2.5 20pm device S 2.0 Im « o a 0.0 0 5 10 15 20 25 30 S k « o a 9 GL 9 O 50um device