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Infrared and Visible Semiconductor Lasers Based on Other Material Systems

This is a continuation from the previous tutorial - Lead-Salt Semiconductor Lasers.


We focused our attention on lead-salt lasers in the previous tutorial. In this tutorial we briefly mention some of the results on semiconductor lasers obtained using material systems other than the lead salts.


1. Infrared Semiconductor Lasers

The wavelength of semiconductor lasers has been extended to about 100 μm using single crystals of Bi1-xSbx solid solutions.

By varying the composition x in the range of 0.08-0.16, the band gap of this material can be varied from 2.7 to 22.6 meV at T = 4.2 K.

For the composition used in laser fabrication, the band gap \(E_\text{g}\) = 12.3 meV corresponded to a wavelength of about 100 μm. The active region was 3.7-mm long, 2.7-mm wide, and 0.17-mm thick. When operated at 4.2 K, the threshold for the laser was 1.38 A.

Attempts have been made to increase the wavelength at which room temperature operation of semiconductor lasers can be realized. These efforts are motivated by the theoretical possibility of an ultra-low-loss fiber at wavelengths exceeding 2 μm.

InGaAsP lasers can provide continuous room-temperature operation up to ~ 1.6 μm. This wavelength can be extended to about 1.8 μm using AIGaAsSb-GaSb double-heterostructure lasers.

Further increase in the wavelength has been realized using the material system InGaAsSb-AIGaAsSb. Using LPE, double-heterostructure lasers were prepared with the quaternary layers lattice-matched to GaSb substrates.

Under pulsed operation of these 2.2 μm devices, the threshold current density at room temperature was ~ 7 kA/cm2. By 1988 this value had been reduced to 1.5 kA/cm2 in AIGaAsSb lasers operating continuously at room temperature.

The threshold current for a 2.34 μm laser was as low as 80 mA through the use of a ridge-waveguide structure.

By 1991 an InGaAsSb semiconductor laser operating at 2.3 μm emitted 900 mW of output power at room temperature under quasi-CW operation; it was necessary to pump it with 200-ns current pulses to avoid heating of the active region. The threshold current density was as low as 1.5 kA/cm2 with a differential quantum efficiency as high as 50%. The threshold current increased exponentially with temperature with a characteristic temperature of about 50 K.

InGaAsSb semiconductor lasers operating at the 3.06-μm wavelength have also been fabricated.



2. Visible Semiconductor Lasers

AIGaAs semiconductor lasers can be designed to operate at wavelengths as short as 780 nm by using a quantum-well structure. However, many applications (for example, optical  data recording and laser printing), require semiconductor lasers operating in the visible region extending from 400-700 nm.

Considerable effort was spent in the late 1980s to develop visible semiconductor lasers with the initial emphasis on red semiconductor lasers operating in the 600-700-nm range, since such lasers could be used in many applications in place of He-Ne gas lasers.

Although the material system GaInAsP-AIGaAs is sometimes used to make red semiconductor lasers, the material of choice for fabrication of such lasers is the quaternary compound InGaAIP formed by replacing a fraction of Ga atoms by Al in the ternary compound In0.5Ga0.5P. Its composition is written as In0.5(Ga1-xAIx)0.5P, where x represents the fraction of Al atoms.

The bandgap \(E_\text{g}\) and the refractive index \(\mu\) of the quaternary compound InGaAlP vary with the fraction \(x\) as



where \(x\) is limited to be in the range 0-0.7 since InGaAlP has an indirect band gap for \(x\) > 0.7.

Both active and cladding layers are made of the same compound by changing the fraction \(x\) and are grown epitaxially over the GaAs substrate. The fraction \(x\) is generally chosen to be 0.7 for the cladding layer in order to maximize the band-gap discontinuity at the heterostructure interface. The active-layer composition depends on the laser wavelength and is typically in the range \(x\) = 0-0.5.

InGaAlP semiconductor lasers operating continuously at room temperature in the wavelength range 670-680 nm were first demonstrated in 1985 by using InGaP active layers (x = 0).

Since then, the performance of such lasers has improved considerably. Output powers as high as 320 m W were obtained in 1990 in a broad-area device. For practical applications it is necessary to obtain the laser output in a single stable spatial mode.

Power levels of up to 50 mW were obtained by using an index-guided structure with a thin layer and optimum facet reflectivities. Power levels of up to 80 m W were later obtained by using a window structure.

Reliability of InGaAIP semiconductor lasers has also improved considerably. Indeed, 670-nm InGaAlP semiconductor lasers emitting 20 mW of output power in a stable transverse mode were available commercially by 1991.

InGaAIP semiconductor lasers emitting in the wavelength region near 630 nm have been developed for applications where He-Ne lasers are commonly used.

It is possible to reduce the operating wavelength of InGaAlP lasers by increasing the Al fraction in the active-layer composition [see Eq. (13-9-1)], and considerable effort was directed toward the realization of 630-nm semiconductor lasers during the early 1990s.

In one scheme, visible lasers operating at 636 nm were demonstrated by using an InGaAIP active layer (with x = 0.15) and a heterobarrier-blocking (HBB) design in which a large band-gap discontinuity is introduced by inserting an InGaP layer between the p-type InGaAIP cladding layer and the p-type GaAs contact layer.

Figure 13.18 shows the L-I characteristics of one such laser at several temperatures. The two insets show the optical spectrum and the far-field patterns when the laser operates at 2 mW. The threshold current of such lasers is relatively large (~ 100 mA) and they are unable to operate at temperatures above 70-80°C.


Figure 13-18.  L-I characteristics of a 636-nm visible laser at several  temperatures. The two insets show the optical spectrum and the far-field when the laser operates at 2 mW. 


In general, the operating characteristics of InGaAIP lasers degrade when the Al composition exceeds 20% (x > 0.2) mainly because the band-gap difference between the active and cladding layers becomes so small that carriers can leak over the heterobarrier [refer to the how to estimate the threshold current density of a semiconductor laser tutorial].

Two techniques have been used to overcome this problem.

First, the wavelength can be reduced by using the quantum-well structure since the photon energy is higher than the band-gap energy through the quantum confinement effect [refer to the quantum well semiconductor lasers tutorial].

Second, it was found that the band gap of an InGaAIP active layer can be increased if the layer is epitaxially grown on a misoriented substrate. The band gap is 50-60 meV wider for InGaAIP active layers grown with a misorientation of the (100)-GaAs substrate by 5-7° toward the [011] direction. This increase in the band gap translates into a 20-30 nm reduction in the wavelength and allows operation near 655 nm even with ternary
InGaP active layers (x = 0).

Figure 13-19 shows the measured wavelengths as a function of the Al fraction x for substrates misoriented by 5 and 7° and compares them with conventional lasers fabricated without any misorientation.

The nearly linear decrease of  wavelength with the Al fraction x implies that the linear relation similar to Eq. (13-9-1) applies even for lasers grown over misoriented substrates.

Figure 13-19 shows the L-J characteristics at several temperatures for a 655-nm laser fabricated with the InGaP active layer (x = 0) deposited over a 7°-misoriented substrate. The maximum operating temperature of 85°C is comparable with the conventional 670-nm semiconductor lasers.

Visible lasers operating near 630 nm can be fabricated by this growth technique by using InGaAIP active layers with only 15% Al content (x = 0.15). The threshold current of one such laser operating continuously at room temperature was 90 mA and the laser could be operated up to 6 mw.


Figure 13-19.  (a) Measured wavelength as a function of the AI fraction x for several visible lasers grown over substrates misoriented by 0, 5, and 7°.


The performance of InGaAIP lasers can be considerably improved by using the multiquantum-well (MQW) design for the active region. The performance is improved even more by using strained MQW lasers. Output powers in excess of 100 mW have been obtained for 670-nm strained MQW lasers. At the same time, such lasers are capable of operating at temperatures as high as 150°C.

The performance of 630-nm is also improved by the adoption of the MQW design. The threshold current of one 636-nm laser was 57 mA. It was capable of emitting up to 15 mW of power although single-mode operation was limited to 8 mW.

The use of strained MQW InGaAIP lasers has provided output powers of up to 50 m W at 632 nm by using an active region with 4 quantum wells of composition In0.53Ga0.47P (x = 0). The laser was capable of providing 30 m W of output power at 50°C. The maximum operating temperature for this laser was more than 100°C since the laser emitted 40 mW even at 80°C.

These lasers also appear to have excellent reliability. The aging characteristics of 12 lasers over 5,500 hours of 3-mW operation at 50°C indicated an estimated mean time to failure of more than 10,000 hours.

The output power can be increased by using an array design. A 1-cm bar of laser arrays operating at 633 nm provided up to 3 W of output power under CW operation at room temperature.

It is possible to reduce the wavelength further by increasing the misorientation angle of the substrate and/or the Al content of the active layer. The wavelength was reduced to 607 nm in one laser in which a 10-nm-thick InGaP active layer (x = 0) was grown over a 15°-misoriented substrate to provide a single strained quantum well.

This laser also incorporated 300-μm-Iong Bragg mirrors on each side of the 300-μm-Iong active layer, and therefore acted as a DBR laser [refer to the DBR lasers tutorial]. It maintained a single-longitudinal mode up to 1.8 times above threshold when operated at 140 K.

Even shorter wavelengths can be obtained by such techniques. Yellow and green semiconductor lasers operating at 576 and 555 nm have been demonstrated, but both of them required low-temperature operation. In general, room-temperature operation of InGaAIP laser appears to be difficult when the wavelength is reduced considerably below 630 nm.

A totally different approach has been followed to realize visible semiconductor lasers in the blue-green region near 500 nm. Semiconductors such as CdS, CdSe, ZnS, ZnSe belong to the II-VI class and have wide band gaps in excess of 2 eV.

Semiconductor lasers made by using these semiconductor materials are expected to emit blue light in the optical region. However, fabrication of such lasers has been hindered by the presence of strain-induced defects during the epitaxial growth process and by the difficulty of making a low-resistance ohmic contact on the epitaxially grown structure.

A sustained development effort during the early 1990s has partially solved these problems. By 1992, ZnSe-based MQW semiconductor lasers were capable of emitting ~0.1 mW of output power in the blue region near 490 nm provided they were cooled to 77 K.

The emission wavelength increased with an increase in the operating temperature, and the laser emitted in the green region near 510 nm for temperatures in excess of 200 K. The threshold current density increased and the differential quantum efficiency declined as the temperature was increased above 77 K.

The threshold current density of one laser increased from 0.4 kA/cm2 at 77 K to 1.5 kA/cm2 at room temperature.

ZnSe lasers generally require the use of the MBE growth technique. The laser structure is grown over a GaAs substrate. The active region consists of MQWs of the ternary compound Zn1-xCdxSe (with x ~ 0.2). The cladding layers are made of ZnSe or the ternary compound ZnSe1-xSx; the addition of sulfur decreases the laser wavelength by increasing the hole confinement energy.

A DFB ZnCdSe laser has also been demonstrated by pumping it optically at room temperature. Further development should make room-temperature operation of electrically pumped ZnSe lasers possible.



The next tutorial discusses about degradation and reliability of semiconductor lasers.

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