T. Camps, C. Bringer, V. Bardinal, G. Almuneau, C. Amat, E. Daran, J.B. Doucet, P. Dubreuil and C. Fontaine A simple and novel design for power monitoring, integrated in a VCSEL, is presented. A Schottky photodiode, placed close to the VCSEL, enables delivery of a photocurrent of several hundred mA from the lateral emitted light, throughout the whole light-current characteristics. It is shown that the Schottky contact significantly reduces the parasitic current in the cavity.
Introduction: Integrated detection in vertical cavity surface emitting lasers (VCSELs) is an important issue to maintain the advantages related to the small dimensions of these laser devices [1]. The solutions appropriate to edge emitter lasers are not relevant for VCSELs, because of their geometry, and especially for arrays. The light power control in VCSELs has therefore been extensively studied, and numerous solutions have been proposed, by an external monitoring through a separate discrete photodiode inserted in the packaging [2], or with a photodiode integrated on the wafer surface [3, 4], i.e. in both cases by monitoring the back reflected laser output beam. The integration of a photodetecting section in the vertical structure has also been proposed, but requires dedicated design and additional steps in the fabrication process [5]. We have recently proposed to use an adjacent VCSEL to photodetect the part of guided light emitted in the plane of the cavity [6]. This signal comes mostly from the spontaneous emission escaping laterally from the VCSEL, but nevertheless can be efficiently used to monitor the normal laser output power, since both signals are monotonously correlated. In this Letter, we present a new configuration using a ring-shaped Schottky photodiode (SPD) which enables to maximise the light detection around the VCSEL. Description of device: Fig. 1 represents the microscopic top view of the VCSEL=Schottky detector device and the corresponding AB cross-section. Compared to a standard oxide-confined VCSEL, the mesa etch is stopped at the top – instead of below – of the nonintentionally doped (NID) zone of the cavity. An annular Schottky Ti=Au contact is deposited on this zone. Thereafter, all the results presented in this Letter correspond to a distance (L) of 4 mm, unless mentioned. To isolate the complete device, a second etch is performed outwards of the SPD ring.
adding a reverse voltage (VS). To prevent any change of the VCSEL characteristics (i.e. IE ’ IA) and to detect more efficiently the lateral emission (i.e. ID ’ IPH), the parasitic current IP has to be minimised; it can be expressed as the following: IP ¼
VE VS RP
ð3Þ
In reverse conduction, the Schottky contact induces a potential (VS) very close to the voltage across the VCSEL PIN diode (VE), which reduces the voltage drop across RP and consequently limits the parasitic current Ip. Increasing the distance L reduces this parasitic current, but also involves a significant reduction of the detectable light intensity. Measurements: Fig. 3 represents the evolutions of the applied voltage (VA), the detected current ID and the emitted power PL against applied current IA. lateral detector
VCSEL
IA
L Rtop
RP
VS
IP
ID
VA VE
A
IPH
IE
Fig. 2 Equivalent electrical model of device VA , V
L, mW
ID, mA
High sensitivity integrated lateral detection in VCSELs
1.5
6
3
1.0
4
2
0.5
2
1
0 50
0
0 0
10
20
30
40
IA, mA
Fig. 3 L(I) and V(I) characteristics of VCSEL and ID(I) characteristic of SPD A
IA
B
1
10
L
L = 4 mm
ID
L
optical waveguide
IA
1.5
DBR P
I DBR N
0
10
ID
-1
10
-2
1.0
10
IPH
-3
10 560 mA
L, mW
VCSEL
current, mA
detector
-4
10
0.5
-5
10
Fig. 1 Top view and AB cross-section of VCSEL=Schottky detector structure
-6
0 1.5
1.39 V
1.74 V
2.0 VA , V
2.5
2.69 V
10 3.0
The electrical equivalent model of the device is illustrated in Fig. 2. The current applied to the VCSEL (IA) is divided into two paths, accordingly to the equation:
Fig. 4 Photocurrent evolution illustrating different regimes in VCSEL operation
IA ¼ IE þ IP
The evolutions of the VA(IA) and PL(IA) characteristics are standard, but the detected current ID(IA) curve exhibits a peculiar shape. The signal detected by the SPD can be attributed to spontaneous emission, since this latter is distinguishable by a wide electroluminescence spectrum [6, 7]. When the gain clamping is expected at the laser threshold, a slope break on the ID curve at low injection is observed [8]. Further, when thermal rollover appears at high injection, the curve is marked by a second slope break. Moreover, the increasing monotonous variation of ID between the threshold and the extinction makes this lateral detection suitable for integrated VCSEL power monitoring. For the low values of IA, the voltage drop in the resistance Rtop (top Bragg mirror) is reduced, consequently the effective voltage VE supported
ð1Þ
where IE corresponds to the effective current passing through the PIN VCSEL diode, and IP to the parasitic current due to the conduction in the NID layer channelling towards the SPD electrode. Measurements demonstrate that the conduction in this layer is purely resistive and can thus be represented by a resistance in our model (RP). In the same way, the detected current through the SPD (ID) can be defined by: ID ¼ IPH þ IP
ð2Þ
where IPH corresponds to the photocurrent related to the lateral detection of the in-plane spontaneous emission. Moreover, the Ti=Au contact on the Al0.3Ga0.7As cavity layer (NID) forms a Schottky barrier,
ELECTRONICS LETTERS 3rd February 2005
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by the intrinsic PIN structure is close to the applied voltage VA. Thus, VA was used as variable to compare the evolutions of the applied and detected currents relative to the emitted power PL, as shown in Fig. 4. The semi-logarithmic scale of the PL(VA) curve helps to distinguish three regimes. First, below VA < 1.39 V, the currents IA and IP are equal, showing that below the electrical threshold of the vertical PIN structure (VCSEL), the applied current is entirely collected by the lateral detector. The detected power PL corresponds in fact to the dark current of the external photodiode used in the measurement setup. Above the electrical threshold of the PIN diode, spontaneous emission starts hopping, following the increasing carrier density. Then the laser reaches the optical threshold around VA ¼ 1.74 V, leading to a slope break in the curve ID , related to the clamping of the carrier density. It can be assumed that beyond the electrical threshold of the PIN diode, the intrinsic voltage VE slowly increases and moreover, this increase is compensated by the concomitant VS increase. The IP variation can then be neglected and its value will be equal to the IA value at electrical threshold (1.39 V). The photocurrent evolution can be easily extracted from the following equation: IPH ¼ ID IP
ð4Þ
The maximum value of IPH is found to be equal to 960 mA for L ¼ 4 mm, 395 mA for L ¼ 8 mm and 170 mA for L ¼ 15 mm. Moreover, the efficiency of the Schottky contact has been successfully tested for several temperatures comprised between 25 and 100 C.
Conclusion: We propose a novel and simple detector device integrated in a VCSEL. This lateral detector comprises a Schottky contact on the cavity. It exploits the optical guiding in the plane of the cavity while lowering the parasitic current. A significant value of photocurrent (>100 mA) is obtained and the slope variations of the collected current can be correlated to VCSEL threshold crossing and extinctions without the need of any signal treatment. This device is suited for used in microsystems as well as continuouswave power monitoring. # IEE 2005 Electronics Letters online no: 20057935 doi: 10.1049/el:20057935
T. Camps, C. Bringer, V. Bardinal, G. Almuneau, C. Amat, E. Daran, J.B. Doucet, P. Dubreuil and C. Fontaine (LAAS-CNRS, 7 Av. du Colonel Roche, 31077 Toulouse cedex 4, France) E-mail:
[email protected] References 1
2 3
4 5
6
7 8
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